Bull. Natl. Mus. Nat. Sci., Ser. B, 34(2), pp. 63–73, June 23, 2008

A Taxonomic Study of Podostemaceae of Japan
Masahiro Kato
Department of Botany, National Museum of Nature and Science, Amakubo 4–1–1, Tsukuba 305–0005, Japan

Abstract A taxonomic revision, including a key to the species, synonymy and taxonomic notes,of the two genera and six species Podostemaceae of Japan is presented. Cladopus is represented by
C. doianus and C. fukienensis, and Hydrobryum, by H. japonicum, H. floribundum, H. puncticulatum, and H. koribanum. Of these, H. floribundum, H. puncticulatum, and H. koribanum are endemic to Japan, while the others are distributed in China (Fujian, Yunnan) and also northern Thailand.
Key words : Cladopus, Hydrobryum, Japan, Podostemaceae, taxonomy.

Introduction
Podostemaceae of Japan were discovered forthe first time from Kubuki River, a tributary of
Sendai River, at Yamazaki, Kagoshima prefecture, southern Kyushu by Imamura (1927). Imamura (1927, 1928a, 1928b) described a new
species Cladopus japonicus Imamura and subsequently another new species assigned to Hydrobryum, H. japonicum (Imamura, 1929). Concomitantly, Koidzumi (1927, 1929, 1931, 1934)
described four species, Lawiella (Cladopus)
austrosatsumensis Koidz. (1931), L. doiana
Koidz. (1927), Hydrobryum floribundum Koidz.
(1929), and H. puncticulatum Koidz. (1934), allfrom Kagoshima prefecture, southern Kyushu.
As a consequence, Koidzumi (1935) recognized
six species of two genera from Japan. Later, twoother species were added: one is Cladopus austro-osumiensis Kadono et N. Usui from Kagoshima prefecture and the other is Hydrobryum koribanum Nakayama et Minamitani from Miyazaki prefecture northeast of Kagoshima prefecture.
In total, two genera and eight species are known
so far (Fig. 1).
In a comparative study of Japanese and Chinese species of Podostemaceae, Shin (1954a)
synonymized Lawiella chinensis (H.C. Chao)
Koidz. of Fujian province under L. kiusiana
Koidz. (C. japonicus Imamura) and suggested
that L. fukienensis (H.C. Chao) Koidz. of Fujian

is probably conspecific with L. doiana Koidz.
[C. doianus (Koidz.) Ohwi]. In the Flora of
China, Wu (1988) recognized three genera andthree species, synonymizing Cladopus chinensis
and C. fukienensis under C. nymanii H. Möller.
Wu’s treatment (Wu 1988) was based on van
Royen’s (1965) opinion that the Japanese C.
japonicus and C. doianus are conspecific with C.
nymanii, which has consequently been recognized as a remarkably widely distributed andpolymorphic species. In her taxonomic treatmentof Podostemoideae (Podostemaceae in her sense)
in Asia, Cusset (1992) also reduced C. chinensis
and C. fukienensis to C. nymanii.
Kato and Kita (2003) examined Chinese Podostemaceae, comparing them with species of
the neighboring regions including Japan. Cladopus japonicus was regarded as conspecific with
C. austrosatsumensis and C. chinensis. Cladopus
doianus was also compared with C. japonicus.
Kato and Kita (2003) compared C. austro-osumiensis and Chinese C. fukienensis.
My herbarium and field studies show that twoof the four Japanese species of Cladopus are conspecific with Chinese species. Furthermore, Hydrobryum japonicum is not endemic to Japan anddistributed much more widely than considered sofar. In this paper I revise the taxonomy of Podostemaceae of Japan, based on morphology andmolecular analysis (Kato and Kita, 2003; Kita
and Kato, 2004; Kato, 2004). All species are re-64Masahiro Kato

ferred to subfamily Podostemoideae.

Key to the Species
1.

Stamen 1, simple; capsules globose, smooth, erect; bracts digitate, surface rough; roots compressed, subcylindrical or ribbon-like, with tufts of leaves at sinuses of root branches (genus
Cladopus).
2. Roots 2–4 mm or more wide, ribbon-like, branching monopodially; bract-bearing floral axes
3–10 mm long ..............................................................................................1. Cladopus doianus
2. Roots to 1(–1.3) mm wide, compressed, subcylindrical, branching sympodially; bract-bearing
floral axes 3–6 mm long..........................................................................2. Cladopus fukienensis
1. Stamens 2, forked from common andropod; capsules ellipsoid, compressed, ribbed, appressed toroots; bracts boat-shaped, entire; roots crustose, with tufts of linear leaves scattered on dorsal surface (genus Hydrobryum).
2. Roots thin (0.1–0.2 mm) ..................................................................4. Hydrobryum floribundum
2. Roots thick (0.2–0.5 mm).
3. Roots 0.3–0.5 mm thick, ovary-septa longitudinally fissured in distal part..........................
....................................................................................................3. Hydrobryum japonicum
3. Roots 0.2–0.4 mm thick; ovary-septa not fissured.
4. Roots soft-chartaceous, not coarse (but coarse in plants on vertical bank-wall ofdam), green or light green; ovules 23–29 per ovary; capsule ca. 10-ribbed ..................
.......................................................................................5. Hydrobryum puncticulatum
4. Roots subcoriaceous, coarse, deep green or dull brownish green; ovules 27–36 perovary; capsule ca. 12-ribbed................................................6. Hydrobryum koribanum

Cladopus H. Möller
Ann. Jard. Bot. Buitenzorg, ser. 2, 16: 115, 1899;
Engl., Nat. Pflanzenfam. 2nd ed. 18a: 50, 1930;
Steenis, Fl. Males. Ser. 1, 4: 65, 1949; Backer et
Bakhuisen van den Brink, Bekn. Fl. Java 1: 204,
1963; Ohwi, Fl. Jap. 394, 1965; C. Cusset, Fl.
Cambodge, Laos et Viêt-nam 14: 71, 1973; Bull.
Mus. Natl. Hist. Nat., B, Adansonia 14(1): 20, f.
1, 2, 1992; T.-L. Wu, Fl. Reipubl. Popularis Sin.
24: 4, 1988; Kato et Kita, Acta Phytotax. Geobot.
54: 90, 2003; Kato, Acta Phytotax. Geobot. 57:
30, 2006. Type species—Cladopus nymanii H.
Möller.
Lawiella Koidz. in Doi, Florula Satsum. 1(2):21, 1927; emend. Koidz. in Doi, Florula Satsum.
2: 94, 1931; Acta Phytotax. Geobot. 4: 23, 1935;
H.-C. Chao, Contr. Inst. Bot. Natl. Acad. Peiping
6(1): 5, 1948 (publ. 1949). Type—Lawielladoiana Koidz.

Hemidistichophyllum Koidz. in Doi, Florula Satsum. 1(3): 24, 1928. Type—Hemidistichophyllum
japonicum (Imamura) Koidz.
Lecomtea Koidz. in Doi, Florula Satsum. 1(4):52, 1929. Type—Lecomtea pierrei (Lecomte)
Koidz.
Notes. In Cusset’s (1992) classification, sect.
Cladopus consists of two species, the widely distributed C. nymanii s. lat. and C. taiensis C. Cusset endemic to Thailand, while sect. Griffithella
consists of C. pierrei (Lecomte) C. Cusset of
Laos and Vietnam and C. hookerianus (Tul.) C.
Cusset of southern India. Other authors (e.g.,
Ohwi, 1965; Chao, 1980; Kadono, 1994; Kadono
and Usui, 1995; Kato and Kita, 2003) recognized
two species (C. chinensis, C. fukienensis, C.
japonicus, C. fukienensis) from China and four
(C. austro-osumiensis, C. austrosatsumensis, C.
doianus, C. japonicus) or three species excluding
C. austrosatsumensis from Japan. Shin (1954a)

Podostemaceae of Japan

treated C. chinensis as conspecific with C. (asLawiella) japonicus. This treatment was followedby Kadono (1994) and Kato and Kita (2003). As
a consequence, no species of Cladopus is endemic to Japan. Shin (1954a) also considered that C.
fukienensis is probably conspecific with C.
doianus.
The type and sole species of Torrenticola, T.
queenslandica, from northeastern Queenslandand southeastern New Guinea, was treated as
Cladopus queenslandicus (Domin) C.D.K.Cook
et Rutish. (Cook and Rutishauser, 2001), andKita and Kato (2001) found it to be nested within
the Cladopus clade in a molecular phylogenetic
tree, although the two genera differ considerably
in vegetative and reproductive characters (Aston,1990). Cladopus may therefore consist of about
10 species. In molecular trees of section Cladopus (Kita and Kato, 2001, 2004; S. Koi, unpubl.
data), there are two clades, one consisting of65

Malesian and Thai species, and the other including the Chinese and Japanese species. The taxonomy of Malesian species will be published in a
separate paper (Kato, in press).
Kato (2006) proposed a new genus Paracladopus, sister to Cladopus, with a new species P. chiangmaiensis M. Kato from northern Thailand.
Subsequently Koi et al. (2008) described a second species P. chanthaburiensis Koi et M. Kato.
Paracladopus chanthaburiensis is similar to
Cladopus in the digitate bracts, while similar to
Hydrobryum and others in the ribbed capsule.
1. Cladopus doianus (Koidz.) Koriba (Figs. 2–4)
J. Jap. Bot. 5: 85, 1928; Ohwi, Fl. Jap. 394, 1965;
Shin in Satake et al. (eds.), Wild Fl. Jap. Herb.
Pl. 2: 214, 1982.—Lawiella doiana Koidz. in
Doi, Florula Satsum. 1(2): 22, 1927. Type—Japan: Manose River, Kagoshima Pref. (Kyushu),
Y. Doi s.n. Sept. 1927 (KYO?, not found).
Cladopus japonicus Imamura, J. Jap. Bot. 5:

Fig. 1. Map of southern Kyushu showing distribution ranges of Japanese species of Podostemaceae. Open circles indicate Cladopus, and solid circles indicate Hydrobryum. Cd, Cladopus doianus; Cf, Cladopus
fukienensis; Hf, Hydrobryum floribundum; Hj, Hydrobryum japonicum; Hk, Hydrobryum koribanum; Hp,
Hydrobryum puncticulatum. K, Kagoshima prefecture; M, Miyazaki prefecture; Y, Yakushima Island.

66

Masahiro Kato

60, t. 1–3, f. 1–4, 1928; Bot. Mag. Tokyo 42:379, pl. 5, 6, 1928; Ohwi, Fl. Jap. 394, 1965;
Shin in Satake et al. (eds.), Wild Fl. Jap. Herb.
Pl. 2: 214, 1982; Kato et Kita, Acta Phytotax.
Geobot. 54: 91, 2003.—Hemidistichophyllum
japonicum (Imamura) Koidz. in Doi, Florula Satsum. 1(3): 24, 1928.—Lawiella kiusiana Koidz.,
Fl. Symb. Orient. Asia. 96, 1930; in Doi, Florula
Satsum. 2: 97, 1931; Shin, J. Jap. Bot. 29: 73,
1954; syn. nov. Type—Japan: Sendai River,Kagoshima Pref. (Kyushu), Imamura, Koriba
and Miki s.n., Aug. 1927 (TI?, not seen).
Lawiella austrosatsumensis Koidz. in Doi,
Florula Satsum. 2: 101, 1931; Acta Phytotax.
Geobot. 4: 26, 1935.—Cladopus austrosatsumensis (Koidz.) Ohwi, Fl. Jap. 394, 1965; Shinin Satake et al. (eds.), Wild Fl. Jap. Herb. Pl. 2:214, 1982; syn. nov. Syntypes—Japan: MawatariRiver, Kagoshima pref. (Kyushu), Doi s.n., June
1929, Doi s.n., Oct. 1929 (KYO?, not seen).
Lawiella chinensis H.C. Chao, Contr. Inst.
Bot. Natl. Acad. Peiping 6: 4, pl. 3, f. 1–19, 1948
(publ. 1949).—Cladopus chinensis (H.C. Chao)
H. C. Chao, Fl. Fukien 1: 481, pl. 437, f. 1–13,
1982; syn. nov. Syntypes—Fujian Province,
China: Ting River, Hukou, Changting Hsien, H.C. Chao 2289 (AU?), Jan. 21, 1945, H.-C. Chao2294 (AU?, IBSC!, KUN!, PE!), Nov. 15, 1945;
Ting River, Lipifeng, H.-C. Chao 2295 (AU?,PE! as 2290), Nov. 15, 1945; Ting River, Shihlipu, H.-C. Chao 2284 (AU?), Oct. 25, 1944, H.C. Chao 2287 (AU?), Nov. 20, 1944, H.-C. Chao2301 (AU?), Jan. 27, 1946; Changantein, Ting
River, H.-C. Chao 2286 (AU?), Nov. 1944; Choshui, Chotien, H.-C. Chao 2297 (AU?), Dec. 13,
1945 (specimens not found at AU; see Kato andKita [2003]).
Cladopus nymanii auct. non H.Möller: T.-L.
Wu in Fl. Reipubl. Popularis Sin. 24: 5, 1988; C.
Cusset, Bull. Mus. Natl. Hist., B, Adansonia
14(1): 22, 1992, p. p.
Distribution. Japan (Kagoshima Pref., Miyazaki Pref., southern Kyushu), southeasterm China(Fujian prov.: Changting, Nan’an).
Habitat. On submerged rocks in rapids in open
places. Roots green.

Notes. Molecular data indicate that there is nodifference in the matK sequence between Cladopus doianus and C. japonicus (Kita and Kato,
2004). Morphologically, plants in Manose River,or C. doianus s.s., may differ from C. japonicus
only in the narrower (to 2.5 mm) roots and thelonger [4–8(–10) mm long] floral axes with theupper bracts larger than the lower. Cladopus
doianus s.s. shows a remarkable teratologicalvariation in the flowers (Doi, 1929; Shin, 1950,
1954b, 1964). Here I treat the two as conspecific.
I am disappointed to synonymize C. japonicus
described by Imamura (1928) who discovered
podostemads from Japan for the first time andunraveled the curious morphology and ecologyof this rare species in his detailed observations.
Cladopus austrosatsumensis, which was considered to be endemic to southern Kyushu, is also
conspecific with C. doianus, as noted by Katoand Kita (2003). The two species do not obviously differ in vegetative or floral characters. Although the leaves of C. austrosatsumensis are notdeciduous and those of C. doianus are deciduous(Koidzumi, 1935), this inconspicuous difference
may be ecological or local.
Although Cladopus japonicus (C. doianus),
like C. fukienensis, has been regarded as conspecific with C. nymanii (Cusset 1992), it is distinct
from C. nymanii with its bract-bearing floral axes
up to 10 mm long, stamens to 2.5 mm long andlonger than the ovaries (vs. to 1.3 mm long andequaling or shorter than the ovaries in C.
doianus). Cladopus doianus also differs from C.
fukienensis by the features used in Key.
In a detailed comparison of the Chinese andJapanese species, Shin (1954a, 1954b) foundconsiderable variation in the stigma, the numberof lobes of the digitate bracts, and the length of
the floral axis, and consequently no significant
difference between C. chinensis and C. japonicus(as Lawiella kiusiana). He concluded that thetwo species are conspecific. His treatment is supported by morphological and molecular data. The
matK sequences are identical in C. doianus and
C. austrosatsumensis (Kita and Kato, 2004).
Cladopus doianus thus defined is not endemic toPodostemaceae of Japan

Japan, but disjunct between Fujian (China) andKyushu (Japan).
Recently this species was discovered in
Miyazaki prefecture as the northernmost population in the distribution range. It grows on a flat
concrete bottom in a channel for agricultural
water running in villages. Our field survey could
not find natural populations in nearby rivers. The
growth in this artificial environment provides aclue for protection of endangered Podostemaceae(see also Notes under Hydrobryum puncticulatum).
2. Cladopus fukienensis (H. C. Chao) H. C.
Chao(Figs. 5–6)
Fl. Fukien 1: 480, pl. 436, f. 1–13, 1982; Kato et
Kita, Acta Phytotax. Geobot. 54: 92, 2003.—
Lawiella fukienensis H. C. Chao, Contr. Inst.
Bot. Natl. Acad. Peiping 6: 4, pl. 2, f. 1–21, 1948
(publ. 1949). Syntypes—Fujian Province, China:
Ting River, Lungmen, Chanting, H.-C. Chao2281 (AU?), Feb. 6, 1944, H.-C. Chao 2282
(AU?, PE!), July 18, 1944, H.-C. Chao 2283
(AU?, IBSC!, KUN!, PE!), Sept. 25, 1944 (fr.),
H.-C. Chao 2292 (AU?, FNU!, PE!), Oct. 30,
1945 (fl., fr.); Ting River, Shijenshia, H.-C. Chao2293 (AU?), Oct. 30, 1945 (specimens at AU not
found; see Kato and Kita [2003]).
Cladopus austro-osumiensis Kadono et N.
Usui, Acta Phytotax. Geobot. 46: 133, f. 1, 2,
1995; syn. nov. Type—Japan: Okawa River,Tashiro-cho, Kagoshima Pref., Kyushu, N. Usuiand M. Mukoyama s.n., Dec. 27, 1990 (OSA).
Cladopus nymanii auct. non H. Möller: T.-L.
Wu in Fl. Reipubl. Popularis Sin. 24: 5, 1988; C.
Cusset, Bull. Mus. Natl. Hist. Nat., B, Adansonia
14(1): 22, 1992, p. p.
Distribution. Japan (Kagoshima Pref., southern Kyushu), southeast China (probably central
Guangdong and Fujian: N of Fuzhou, Changting).
Habitat. On seasonally submerged rocks instreams and rivers.
Notes. Chinese plants of Cladopus fukienensis
share with C. austro-osumiensis of Japan in the
narrow (usually 0.5–1 mm wide), subcylindrical67

roots and rather long (up to 6 mm; to 8 mm in C.
austro-osumiensis) bract-bearing floral axes. As
noted in Kato and Kita (2003), here I have thetwo conspecific. This treatment is in accordancewith molecular evidence (Kita and Kato, 2004; S.
Koi, unpubl. data). There are differences in only
a few characters, however. In C. fukienensis thestalk of the capsule is 1.3–3 mm long and extrudes from the spathella, while in C. austro-osumiensis the stalk of the capsule is 0.5–2 mmlong and is often surrounded by the spathella in
the lower half.
Hydrobryum Endl.
Gen. Pl. 1375, 1841, p.p.; Tul., Ann. Sci. Nat.
ser. 3, 11: 103, 1849; Arch. Mus. Paris 6: 137,
1852; Hook. f., Fl. Brit. Ind. 5: 63, 1886; Willis,
Ann. Roy. Bot. Gard. (Peradeniya) 1(3): 225,
1902; Engl., Nat. Pflanzenfam., 2nd ed. 18a: 52,
1930; Ohwi, Fl. Jap. 393, 1965; C. Cusset, Fl.
Cambodge, Laos et Viêt-nam 14: 66, 1973; Bull.
Mus. Natl. Hist. Nat., B, Adansonia 14(1): 44,
1992; D.-D. Tao, Acta Bot. Yunnan. 5: 175,
1983; T.-L. Wu, Fl. Reipubl. Popularis Sin. 24: 4,
1988; Kato et Kita, Acta Phytotax. Geobot. 54:
94, 2003; Kato, Acta Phytotax. Geobot. 57: 143,
2004. Type species—Hydrobryum griffithii(Wall. ex Griff.) Tul.
Hydroanzia Koidz., Acta Phytotax. Geobot. 4:
26, 1935. Lectotype species—Hydroanzia floribunda (Koidz.) Koidz.
Notes. In Asia, all species of Hydrobryum,
Hanseniella (2 species), Thawatchaia (1species), Zeylanidium olivaceum (Gardner)
Engl., and Z. maheshwarii C. J. Mathew etSatheesh, have crustose roots with tufts of leaves
scattered over the dorsal surface of the root. The
foliose roots are the most specialized transformation from the typical cylindrical root, and tufts ofleaves on reduced, inconspicuous shoot are also
extremely reduced shoots (Ota et al., 2001). Our
recent field and molecular phylogenetic studies
have shown that Hydrobryum of Laos includes
species devoid of crustose roots (S. Koi and M.
Kato, unpubl. data). The definition of the genus

68

Masahiro Kato

Figs. 2–9. Cladopus and Hydrobryum of Japan. Figs. 2–4. Cladopus doianus. Fig. 2. Ramified roots 2–4 mm
wide, creeping on rock surface. Shoot buds are borne at sinuses of root branches. Photo by S. Koi. Figs. 3, 4,
flowers with single stamen above digitate bracts. Scale unit on right side of Fig. 4 is 1 mm. Photos by T. Minamitani. Figs. 5–6. Cladopus fukienensis. Fig. 5. Root up to 1 mm wide ramified and meandering. Tufts oflinear leaves are borne at sinuses of root branches. Photo by N. Katayama. Fig. 6. Flowers (with ovaries ca.
1.5 mm long) above digitate bracts. Photo by M. Kato. Figs. 7–9. Hydrobryum japonicum. Fig. 7. Crustoseroot, 5–15 cm wide, adhering to rock surface. Linear leaves are borne on dorsal surface. Photo by N. Katayama. Fig. 8. Flower buds above linear leaves and ovate bracts. Photo by N. Katayama. Fig. 9. Young capsules(ca. 2 mm long) with vertical stripes, which become ribs as fruits ripen. Photo by M. Kato.

Podostemaceae of Japanrequires substantial amendment.
Koidzumi (1935) distinguished Hydroanzia
from Hydrobryum by its perennial habit. However, plants of Hydrobryum and other genera may
be annual or perennial according to where theygrow. Plants exposed to air may become desiccated and die in dry seasons if they occur on upper
rocks, but submerged plants may remain alive
even in the dry season.
In his taxonomic treatment of Podostemaceaeof Thailand, Kato (2004) described 10 species ofHydrobryum from Thailand. Hydrobryum japonicum, one of them, occurs in northern Thailand,
whereas most others are endemic to Thailand.
3. Hydrobryum japonicum Imamura (Figs. 7–9)
Bot. Mag. Tokyo 42: 376, 1928; Bot. Mag.
Tokyo 43: 332, 1929; Ohwi, Fl. Jap. 394, 1965;
Shin in Satake et al. (eds.), Wild Fl. Jap. Herb.
Pl. 2: 214, 1982; C. Cusset, Bull. Mus. Natl.
Hist. Nat., B, Adansonia 14(1): 46, 1992, p. p.;
Kato et Kita, Acta Phytotax. Geobot. 54: 95,
2003; Kato, Acta Phytotax. Geobot. 55: 167,
2006—Hydroanzia japonica (Imamura) Koidz.,
Acta Phytotax. Geobot. 4: 28, 1935. Type—Japan: Ohnejime, Kagoshima Pref. (Kyushu),Senokuchi s.n. (TI?).
Distribution. South-central China (southern
Yunnan), southern Japan (southern Kyushu),northern and central (Cusset 1992) Thailand.
Habitat. On rocks in streams at forest edges.
Notes. This species has been considered to be
an endemic to Japan until recently (Kadono1994). However, Kato and Kita (2003) and Kato
(2004) revealed that it is distributed in Yunnan,
China, and northern Thailand. Kita and Kato
(2004) argued that “Taking this relationship (i.e.,
H. japonicum is close to Japanese H. floribundum, H. koribanum and H. puncticulatum) into
account, the distribution pattern is unusual in that
H. japonicum is disjunct between Thailand (Figs.
16–17), south-central China and Japan.” Kita andKato (2004) also argued that the disjunct distribution is due to three dispersal events. The earliest
dispersal occurred northward to give rise to a distribution pattern similar to the present one, fol-69

lowed by two events, one dispersal from Japan to
Yunnan and the other most recent dispersal fromJapan to northern Thailand.
A northern Vietnamese plant (Pételot 3370 [P]from Sapa [Chapa], Laocai) with cristate stigmas
is H. griffithii (Cusset, 1973). But Cusset (1992)
misidentified it as H. japonicum. Therefore, there
is no evidence that H. japonicum occurs in Vietnam.
Hydrobryum floribundum Koidz.
(Figs. 10–11)
J. Jap. Bot. 6: 1 cum photos, 1929; Ohwi, Fl. Jap.
394, 1965; Shin in Satake et al. (eds.), Wild Fl.
Jap. Herb. Pl. 2: 214, 1982—Hydroanzia floribunda (Koidz.) Koidz., Acta Phytotax. Geobot.
4: 27, 1935—Hydrobryum griffithii var. floribundum (Koidz.) Koidz., in Doi, Florula Satsum. 2:
100, 1931. Type—Japan: Maegawa River,Shibushi, Kagoshima Pref. (Kyushu), Y. Doi s.n.
(KYO?).
Distribution. Japan (Kagoshima Pref., southern Kyushu).
Habitat. On rocks in streams.
Notes. This is endemic to a small area ofsouthern Kyushu, Japan, where H. japonicum
does not occur together. It grows together with
Cladopus doianus, a rare case compared with allother Japanese species, which occur in single
rivers or river systems. However, co-occurrencesof multiple species are frequent in tropical rivers.
Hydrobryum floribundum is the most similar to
H. japonicum in the leaves 4–9 per tuft, 4–7 mm
long, the bracts 4(–6), the ovary ca. 2 mm long,the septum fissured in the distal part, and theovules 23–32 per capsule. The other Japanese
species have the ovary septum not fissured. Hydrobryum floribundum differs from H. japonicum
in the thickness of the root, as shown in Key:
0.1–0.2 mm in H. floribundum and 0.3–0.5 mm in
H. japonicum. Molecular phylogenetic analyses(Kita and Kato, 2004; S. Koi, unpubl. data) showthat H. floribundum is nested within H. japonicum. Cusset (1992) reduced H. floribundum,
along with H. puncticulatum, under H. japonicum. Because the two are distinct morphologi4.

70

Masahiro Kato

Figs. 10–15. Hydrobryum of Japan. Figs. 10–11. Hydrobryum floribundum. Fig. 10. Thin (0.1–0.2 mm thick),crustose roots adhering on rock surface. Photo by N. Katayama. Fig. 11. Flowers, with two stamens beside ellipsoid ovaries, dense on root. Ovaries are ca. 2 mm long, 1 mm thick. Photo by M. Kato. Figs. 12–13. Hydrobryum puncticulatum. Fig. 12. Crustose roots on rock surface, with hair-like leaves on dorsal surface. Photo
by S. Koi. Fig. 13. Young fruits (ca. 1.5 mm long) on dorsal surface of root. Photo by M. Kato. Figs. 14–15.
Hydrobryum koribanum. Photo by M. Kato. Fig. 14. Dull-brownish green, coarse, crustose root on rock surface. Fig. 15. Flowers at anthesis on dorsal surface of root. Ovaries are ca. 2 mm long.

Podostemaceae of Japan

cally (in the thickness of the root), I consider thatH. floribundum is a local species recently derived
from the widely distributed H. japonicum. If thistreatment is accepted, H. japonicum is a “paraphyletic” species ancestral to H. floribundum.
Hydrobryum puncticulatum Koidz.
(Figs. 12–13)
Acta Phytotax. Geobot. 3: 178, 1934; Ohwi, Fl.
Jap. 394, 1965; Shin in Satake et al. (eds.), Wild
Fl. Jap. Herb. Pl. 2: 214, 1982—Hydroanzia
puncticulatum (Koidz.) Koidz., Acta Phytotax.
Geobot. 4: 29, 1935. Type—Japan: Isso River,
Yakushima Island, Kagoshima pref. (Kyushu), Y.
Doi s.n. (KYO?).
Distribution. Japan (Yakushima Island, southern Kyushu).
Habitat. On rocks in stream (Isso R.).
Notes. This is recorded only from the river on
Yakushima Island. It grows not only on natural
rocks but also on concrete bank surfaces of asmall dam. This artificial environment is useful5.

71

to protect the podostemads, as in the case of
Cladopus doianus.
Kita and Kato (2004) showed that this species
is sister to H. koribanum. This relationship isconfirmed by a molecular analysis with a larger
set of species (samples) (S. Koi, unpubl. data). It
is consistent with morphological data such as theshared non-fissured ovary-septum (Nakayama
and Minamitani, 1999). The phylogeny is interesting in light of distribution, because both
species grow in small areas (one river or river
system) and H. koribanum occurs in Miyazaki
Prefecture separately from Yakushima Island. In
comparison, H. japonicum and H. floribundumare distributed on the Osumi Peninsula, whichlies between the localities of those two species.
Kita and Kato (2004) implies that this distribution pattern may have reflected range disturbances due to frequent volcanic eruptions insouthern Kyushu during the Quaternary, range
contraction due to climate cooling during the
Pleistocene, and/or the disjunction of Kyushu

Figs. 16–17. Hydrobryum japonicum of Thailand. Photo by M. Kato. Fig. 16. Flowers with forked stamens andflower buds (ellipsoids) dense on dorsal surface of crustose root. Fig. 17. Flower with two stamens (anthers,
yellow) connected by common andropod behind ovary (not seen) and ligulate stigmas on top of ellipsoid
ovary (ca. 1.5 mm wide). An ovary is subtended at the base by a breaking spathella.

72

Masahiro Kato

from Asian continent by formation of the East
China Sea.
Cusset (1992) combined both H. puncticulatum with H. japonicum, but this treatment is notsupported by molecular data showing that thethree are deeply diverged (Kita and Kato, 2004;
K. Koi, unpubl. data).
6. Hydrobryum koribanum Imamura ex
Nakayama et Minamitani(Figs. 14–15)
J. Jap. Bot. 74: 314, f. 1, 5, 1999. Type–Japan:
Iwase River, Nishimorokata, Miyazaki pref,
Kyushu, S. Nakayama and T. Minamitani s.n.,Dec. 21, 1996 (TI!).
Distribution. Japan (Iwase River, Oyodo River,Miyazaki pref., southern Kyushu).
Habitat. On rocks in rivers.
Notes. This species has long been noticed tooccur since the discovery by S. Imamura, but was
described rather recently by Nakayama and Minamitani (1999) who made a detailed comparisonof this and other Japanese species.
Acknowledgments
I thank T. Minamitani, S. Koi and N. Katayama for their help on collecting trips and providing photographs, and the directors of the herbaria
KYO and TI for allowing us to access to their
specimens. I am also obliged to M. Hotta who informed me of the detailed localities in Japan of
Podostemaceae at an initial phase of this study.
This study was supported in part by a Grant-inAid for Scientific Research from the Japan Society for the Promotion of Science.
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