Boehmeria japonica

Wilmot-Dear, C. M. & Friis, I., 2013, The Old World species of Boehmeria (Urticaceae, tribus Boehmerieae). A taxonomic revision, Blumea 58 (2), pp. 85-216 : 186-189

28. Boehmeria japonica (L.f.) Miq. — Fig. 34 , 35 ; Map 35–37

Boehmeria japonica (L.f.) Miq. (1867) 131. ― Urtica japonica L.f. (1781) 418. ― Boehmeria platyphylla D.Don var. japonica (L.f.) Wedd. (1869) 213, nom. illeg., non Boehmeria platyphylla D.Don var. japonica Wedd. (1856) 365. ― Type: Thunberg s.n. (lecto in Smith herbarium [Linnaean Society of London], IDC microfiche LINN. no. 1456-5, lectotype selected by Yahara 1984a: 133), Japan. – See Note 1.

Urtica spicata Thunb. (1784) 69. ― Boehmeria spicata (Thunb.) Thunb. (1794b) 330, nom. illeg. superfl. ― Type: Thunberg 22116 (lecto UPS-THUNB, left-hand plant, lectotype selected by Yahara 1984a: 130). – See Note 1.

Subshrub or woody-based herb,often robust and thick-stemmed, 1.5–5 m tall; ultimate stems (1–) 1.5–2 mm diam; hairs very sparse to abundant, fine, adpressed and short or ± spreading and often of various lengths. Stipules long and narrow, 4–9 by 1–2 mm, with long-acuminate apex, with dense pubescence on outside often restricted to midrib. Leaves opposite, not or slightly dimorphic with length of ‘larger’ leaves 1–1.5(–2) × that of ‘smaller’ leaves, ovate to broadly rhombic-ovate or ovate-truncate, rarely obovate, rather variable in size and proportions, small to very large, 2–20 by 1–17 cm, length equalling or up to 2 × width; margin large-dentate, teeth 6–15(–25) large, broad, slightly up-curved, acute, 1–15 by 1–15 mm, distal ones progressively markedly larger, length of those near apex at least twice, usually 3–10 × length of those in proximal third of lamina; leaf apex varying from acuminate with single long tooth (several times longer than most marginal teeth and often constricted at base), to rounded-truncate in outline but 3 – 5-toothed, with 1– 2 long subapical teeth either side of terminal tooth, leaf appearing either deeply laciniate (terminal and subapical teeth narrow, in-curved) or ± 3-lobed (terminal and subapical teeth broad, outward-pointing, often secondarily toothed); base broadly cuneate to ± truncate (rarely slightly cordate), basal veins reaching distal third of leaf, upper lateral veins 3 – 5 but scarcely distinguishable from coarser tertiary venation, lowermost arising near middle of lamina, these and somewhat scalariform coarser tertiary venation visible but not prominent abaxially; texture usually thin- or thick-chartaceous, sometimes ± membranous; hairs on adaxial surface abundant but often minute inconspicuous, adpressed or ± spreading, cystoliths fairly abundant but not conspicuous; hairs on abaxial surface denser but finer mostly restricted to main veins and fine reticulation; petiole very variable with respect to leaf-size, 0.25–0.5 × lamina length. Flower­clusters borne on leafless axes, these unisexual, erect or ± pendulous, (5–) 10–25 cm long, 1(–several) per axil, unbranched or with few long branches near base (rarely throughout most of length); plants unisexual or bisexual but male axes often rare (see discussion in Note 3); flower-clusters rather close-spaced almost contiguous or up to 2 mm apart, male clusters small with few–c.10 flowers, female very variable in number of flowers and size, 1.5–2(–3) mm diam in flower (up to 4–5 mm in fruit), with (10–)30–50 or more densely crowded flowers; bracts conspicuous or inconspicuous, in male clusters short, broadly triangular, up to 1.5 mm long, in female clusters up to 5 mm long, linear-oblong or narrowly triangular. Male flowers 4-merous, sessile, lobes divided more or less to base, mature buds depressed-globose, c. 1.5 mm diam, tepals without or with only slight dorsal appendages (male flowers often absent or infertile as discussed below), hairs sparse, spreading. Female flowers narrowly ellipsoid or ovoid, slightly flattened, c. 1 by 0.2–0.4 mm, hairs abundant, short, stiff; stigma very variable, (0.5–) 1–2.5 mm long. Fruiting perianth rather variable in size and shape, narrowly ellipsoid to broadly obovoid, rarely ovoid, abruptly or gradually tapering to base and without or with very short abrupt apical beak, slightly laterally flattened in middle portion and with distinct marginal rim or (especially Japanese material) markedly flattened into wing around achene, 1–2.5 by 0.5–1.5 with length scarcely exceeding or up to 2.5 × width; with indumentum of sparse or abundant adpressed or long to short velvety spreading hairs especially in distal half. Achene occupying middle third of fruiting perianth (broadly-winged forms) or almost filling fruiting perianth.

Distribution ― China (including Taiwan), South Korea, Japan (Hokkaido, Honshu, Shikoku, Kyushu).

Habitat & Ecology ― Very varied; swamps in sandy soil; streams and moist slopes in deciduous forest, forest margins; open or shady damp ravines; thin evergreen forest; scrub in valleys; barren dry hillsides; grassland; walls and roadsides; 70–2600 m altitude.

Conservation status ― Least Concern (LC). It is known from more than 200 collections from many locations in a wide range of habitats. Formal assessments are not given for the three, intergrading, varieties.

Notes ― 1. The first name to be published on material of this taxon brought back from Japan by Thunberg was Urtica japonica L.f. (1781), based on an unnumbered Thunberg specimen in the Linnaean herbarium, followed by Urtica spicata Thunb. ( Thunberg 1784) , based on 2 collections (comprising 3 specimens) in the Thunberg herbarium. Thunberg later (1794b) transferred his Urtica spicata to Boehmeria and cited Urtica japonica as a synonym; his new combination B. spicata (Thunb.) Thunb. is therefore illegitimate, since Linnaeus’s epithet japonica (1781), is earlier and should have been adopted. Subsequently, Yahara (1984a: 135) stated that the type of U. japonica , (a single specimen) although considered a duplicate of Thunberg 22115 & 22116, is similar only to the left half of 22116; he considered that the other two specimens of Thunberg’s type material belonged to a distinct taxon, for which he maintained Thunberg’s illegitimate name B. spicata . This distinct taxon is recognised here as B. japonica var. tenera taking up the earliest available name at varietal rank.

2. Boehmeria japonica s.lat., as here circumscribed, occurs throughout most of Japan (Hokkaido, Honshu, Kyushu, Shikoku), in South Korea and most of China including Taiwan. It is a herb or subshrub characterised by ovate leaves with marginal teeth markedly up-curved, the distal ones progressively regularly longer and wider than proximal, relatively few even on large leaves, often also a tendency to leaf apex 3 – 5-lobed and leaf base often truncate.

3. Boehmeria japonica exhibits a matrix of variation with respect to a range of leaf characters some of which vary independently of one another: leaf-size and proportions, number of marginal teeth, leaf apex (caudate, acuminate or truncate-laciniate with 1–2 almost equally long marginal teeth or even lobes either side of terminal tooth), indumentum of stems, petioles and inflorescence-bearing axis (spreading or adpressed). As discussed in detail in our introduction section on species delimitation, Okabe (1956) and Yahara (1983a, 1984a) demonstrated apomixis and polyploidy in some parts of this group of taxonomic entities in Japan, complicated by the occasional production of fertile male flowers and subsequent hybridisation within the group or with other Japanese taxa, resulting in “complicated morphological variation and obscure species boundaries” such that “ Boehmeria in Japan is an example of an apomictic complex” (Yahara 1983a: 218). Thus a formal classification is artificial and unsatisfactory, as is well-illustrated by the complicated taxonomic history of this group discussed in the introduction. Yahara’s work also strongly suggested hybridisation with taxa outside the group, B. splitgerbera (previously referred to as B. biloba ) (Yahara 1983a: 243, 1984a: 141) and B. holosericea (Yahara 1984a: 141) .

Interpreting the range of variation seen in herbarium material in the light of the morphological and cytological findings of Okabe and Yahara, this group of entities seems best considered as comprising three partly intergrading varieties, differing in a combination of indumentum and leaf morphology with incompletely overlapping distributions, the morphological distinctions slightly obscured by intermediates between them and by a group of hybrids, some of uncertain parentage.

4. Boehmeria japonica var. japonica is characterised by its spreading pubescence on young stems, inflorescence-bearing axes, petioles and abaxial leaf surface, and its medium or large often thick-textured leaves. Inflorescence-bearing axes vary widely in length and in spacing of clusters. Its leaf shape varies greatly from narrowly rhombic-ovate and acuminate to very wide with apex laciniate-3-toothed or leaf ± 3-lobed, base truncate or broadly rounded, and marginal teeth few and large. It occurs further south and less far north in China than the other two varieties and is absent from northern Japan (Hokkaido). The forms with broad-laciniate leaves appear to extend to higher altitude and more moist habitats than the ovate acuminate forms.

This variety broadly corresponds to Yahara’s B. longispica (1983a) which he subsequently (1984a) altered to “ B. longispica complex” in which he recognised two intergrading varieties, B. japonica var. japonica and var. apiculata , the former with apex long-caudate and distal lateral teeth narrow and long-caudate, the latter more variable with distal lateral teeth broader and apex acute to deeply 3-toothed. However, our concept of var. japonica is broader in including material conforming to the type of B. japonica var. tricuspis (Hance) Maxim. (leaves deeply laciniate, ± 3-lobed to nearly half of lamina length, width of lamina usually exceeding length). The epithet tricuspis is not mentioned by Yahara, but some of his line-drawings of leaves of his B. longispica complex represent almost this extreme variant. Although the type of var. tricuspis represents one extreme of the range of variation, neither this nor Yahara’s concept of var. japonica and var. apiculata are maintained here because abundant herbarium material provides a continuous series of intergrading intermediates exhibiting various character states with respect to leaf proportions, apex and marginal teeth; therefore any separation into two entities would be arbitrary and unworkable. This contrasts with the situation between var. silvestrii and var. tenera discussed below (see Notes 5, 6), where there is a similar difference in leaf apex but where most entities can be referred to one or the other taxon with comparatively few intermediate collections.

Yahara (1983a) demonstrated various polyploid apomictic races and the occasional production of fertile male flowers within his ‘ B. longispica ’. Further cytological investigation and population studies including these extreme ( ‘var. tricuspis ’) entities would be helpful to clarify the situation with respect to apomixis and possible hybridisation with the following variety, var. silvestrii .

5. Boehmeria japonica var. silvestrii has leaves deeply laciniate and few-toothed as in the apically broad-laciniate forms of var. japonica , but thinner-textured, often smaller and the plant is glabrous or sparsely adpressed-hairy. It is the only variety recorded as far north as Hokkaido, and in China it extends further to the north, and less far south and east, than var. japonica . The cytological work of Okabe (1956) showed it to be diploid and sexually reproducing, supporting recognition of it as a distinct entity. Yahara (1983a: 232) discussed a range of polyploid entities intermediate between var. silvestrii and his ‘ B. longispica’ ( = var. japonica as circumscribed here) with leaves laciniate or ± 3-lobed, very large and thick-textured but glabrescent. These entities included material conforming to the type of B. platanifolia . Subsequently, Yahara (1984a: 141) assumed this material to be a hybrid between his ‘ B. japonica ’ ( = var. japonica here) and ‘ B. silvestrii ’ ( = var. silvestrii ). We follow his suggestion here and consider these variants as intermediates between the two varieties here recognised; they are cited under var. japonica , Note 6. The above-mentioned extreme variants of var. japonica with deeply laciniate but spreading-hairy (rather than adpressed-hairy) leaves, to which the epithet ‘ tricuspis ’ applies, may also be of hybrid origin.

6. Boehmeria japonica var. tenera is almost sympatric with var. silvestrii (no material seen from Hokkaido but extending further south to most of southern China, incl. Taiwan). It has inflorescence-bearing axes short and erect and is glabrous or glabrescent with thin-textured leaves, as in var. silvestrii but leaves are always fairly small and narrow, ovate or rhombic-ovate with apex long-acuminate. Leaves are of similar shape to the narrow acuminate forms of var. japonica but mostly much smaller and relatively narrower with fewer marginal teeth, these longer relative to leaf-size, giving a distinctly long-toothed appearance, and the two varieties can be distinguished on a combination of characters as follows. Leaves of var. tenera always have a relatively long terminal tooth and any within the size range of var. japonica (i.e., 6–12 cm long) have an extremely long terminal tooth comprising 0.3–0.5 × total lamina length whereas any narrow and acuminate leaves of var. japonica which are similarly small (only 6–12 cm) have a terminal tooth only 0.17–0.25 × lamina length.

Var. tenera broadly corresponds to Yahara’s two entities first referred to (1983a) as B. gracilis and (confusingly) ‘ B. japonica ’ and later (1984a) as respectively a diploid sexually reproducing race and a triploid apomictic race of ‘ B. spicata ’. Yahara (1983a) demonstrated that these two entities showed partial altitudinal separation and a range of forms intermediate in morphology and distribution which his cytological investigation strongly suggested to be of hybrid origin. An apomictic polyploid was found at low to medium altitude (300–650 m), a much-branched woody perennial (lacking male flowers) with small leaves with relatively few very in-curved marginal teeth. The sexually reproducing diploid (his B. gracilis ) was found at medium to high altitude (600 m and above), a woody-based or annual herb with more numerous less in-curved marginal teeth, shorter terminal tooth and shorter denser hairs on inflorescence-bearing axis. Intermediate plants were found at altitudes of 350 - 800 m. These usually had a few developed male flowers, were very variable morphologically and were found to include a triploid and a tetraploid race (both races apparently including several apomictic clones), suggesting that this complex taxonomic situation had originated through partial introgression between the supposed parental species.

These two entities, apomictic and sexually reproducing, are here treated as two races of the same variety, because the presence of a continuous series of intermediates makes distinction, as Yahara himself admits, “difficult” (and from herbarium material in practice often impossible). The epithet tenera , the earliest available name at varietal rank, is included in the synonymy of B. spicata by Yahara (1984a: 130; note that on page 132 he quotes it as “indistinguishable from typical B. japonica ”, but from the context this must be a slip of the pen for “ B. spicata ”).

7. Most collections of B. japonica with thin leaves glabrescent abaxially can satisfactorily be identified to either var. silvestrii (apex truncate-laciniate, leaves medium) or var. tenera (apex acuminate, leaves small or medium), but a few individuals with intermediate leaf-shapes have also been seen. Material also exist with leaf apex widely varying on the same plant, from long-acuminate near stem apex to truncate-laciniate in lower parts, further illustrating the unsatisfactory nature of formal taxonomy within this partly apomictic group as a whole. Interestingly, var. silvestrii appears to be much less common since more than twice as many collections have been seen conforming to var. tenera .

8. There is fairly clear distinction (based on indumentum, leaf-size, -texture and relative proportions, relative length of terminal tooth and number of teeth) between var. tenera and the acuminate-leaved forms of var. japonica . Only a few intermediate collections (with small sparsely hairy leaves) have been seen compared to the large amount of clearly distinct material of the two taxa. However, between var. japonica and var. silvestrii (which differ in indumentum and leaf texture) there exists a greater range of intermediate entities, as mentioned above (see Note 5); we follow Yahara’s interpretation of such entities (1984a: 141 referred to as B. platanifolia ) as hybrids. Such material is illustrated in Fig 34j, k .

9 . Although not all types of names published for eastern Asian entities within this complex have been seen, the discussions and figures of Yahara (1983a, 1984a) make their identities clear .

10. The presence or absence of dorsal appendages on male tepals, usually consistent within a taxon, is variable in B. japonica , being usually ± absent but in var. japonica and var. tenera sometimes prominent. The variability in this character also suggests a complex situation of hybridisation.

11. The allopatric (East Himalayan) B. ternifolia var. kamley has leaf shape almost identical to the broad-laciniate variant of B. japonica var. japonica , but Acharya et al. (2002, 2003), found no evidence of polyploidy and apomixis within B. ternifolia , demonstrating that B. ternifolia and B. japonica are cytologically distinct as well as allopatric, with no evidence of hybridisation (see further discussion under B. ternifolia , Note 4).

12. Boehmeria sieboldiana (sympatric but absent from northern China and extending to the Ryukyu Islands) is often confused with B. japonica ( var. japonica and var. tenera ), distinguished by leaves rhombic-ovate rather than ovate, with base rounded rather than usually cuneate to truncate, ± glabrous, with distal marginal teeth not or scarcely larger than proximal ones and out-pointing with ± straight upper margin and inflorescence-bearing axis never branched. It is distinguished additionally from var. japonica by leaves always thin-textured, never spreading-hairy, always narrow (length of at least upper leaves 2–4 × width) and from var. tenera by its more numerous relatively short marginal teeth (15–40 either side rather than up to 12); the teeth of var. tenera are often so long relative to leaf-size that the leaf appears almost laciniate, and its leaves are also mostly smaller (mostly less than 10 cm long (rather than mostly over 10 cm) and relatively broader, while any leaves within the size range of B. sieboldiana have a terminal tooth 0.3–0.5 × (rather than up to 0.25 ×) total lamina length. Flowers in B. japonica are also narrower and fruiting perianth is more variable, often much more flattened and tapered.

However, forms of B. japonica intermediate between the various varieties (hairs adpressed, leaves relatively large and narrow, marginal teeth only moderately up-curved) are hard to distinguish from B. sieboldiana var. sieboldiana and several collections somewhat intermediate between the two species have been seen, mostly from Japan; these may be hybrids since (as discussed in the introduction) hybridisation has been demonstrated between B. japonica and other taxa (Yahara 1983a, 1984a). This intermediate material includes the type collections of B. kiusiana and B. sieboldiana var. ovata which are cited below under var. japonica (see Note 7).

13. Boehmeria holosericea , which is sympatric but less widespread, absent from China and extending to the Ryukyu Islands, has large broad leaves and spreading indumentum which can be confused with those of B. japonica var. japonica , differing in marginal teeth more numerous and not up-curved, not or scarcely larger towards leaf apex and often rounded; its apical tooth is often broad and indistinct rather than markedly larger than the marginal teeth; hairs on leaves are soft but coarser, usually longer and its female clusters are often larger and always densely crowded along the axis. However, forms exist with leaf margin characters intermediate between B. holosericea and B. japonica s.lat. These may be hybrids between the two; see further discussion on hybridisation below and under B. holosericea (see Notes 6, 7).

14. Material exists which is intermediate between B. japonica s.lat. and B. splitgerbera . The population studies of Yahara (1983a: 218) strongly suggested hybridisation between the two; his further reflections (1984a: 141) on these studies also suggested hybridisation between B. japonica and B. holosericea , as mentioned above.

15. Two varieties of the widespread B. virgata subsp. macrophylla , the partly sympatric var. strigosa ( China, Indochina ) and var. rotundifolia (eastern Himalaya, China), have marginal teeth mostly increasingly up-curved near leaf apex and can therefore be confused with B. japonica , differing in marginal teeth never also markedly larger near leaf apex. Var. tenera can be confused with the small-leaved variety B. virgata subsp. macrophylla var. densiglomerata which differs in leaf base often cordate, never cuneate, marginal teeth ± uniform and outward-pointing and inflorescence-bearing axes never over 6 cm long. Acharya et al. (2002, 2003), found no evidence of polyploidy or apomixis within Himalayan material of ‘ B. macrophylla ’ (= B. virgata subsp. macrophylla ), and thus found no evidence of any hybridisation with the Japanese and Chinese partly polyploid and apomictic and partly hybridising entities which are here all included within B. japonica s.lat. He suggested that the Himalayan diploid sexually reproducing taxa, which are restricted to warm humid habitats, may be ancestral to the Japanese and Chinese taxa whose polyploidy has enabled them to be tolerant of a wider range of environmental conditions. Further cytological and molecular work, also extending to Indonesian material, would be helpful in clarifying relationships.

Key to varieties

1. Young stems,petioles,inflorescence-bearing axis with spreading pubescence, leaves often similarly hairy on major and minor venation abaxially (adpressed or spreading adaxially); leaves (6–) 8–20 cm long, mostly thick-textured and/or with more than 12 teeth either side; male flowers densely hairy. — China incl. Taiwan; South Korea; Japan (excl. Hokkaido)................................... a. var. japonica

1. Young stems, petioles, leaves and often inflorescence-bearing axis entirely glabrous or with sparse short fine adpressed hairs; leaves membranous or very thin-chartaceous, often <6 cm long, teeth up to 12 either side; male flowers only sparsely spreading-hairy......................... 2

2. Leaf apex very broadly rounded to truncate in outline,with 1(–2) very long teeth either side of terminal tooth giving 3(–5)- toothed-laciniate appearance; leaves very broad, length only 1–1.5 × width. — Northern & central China (excl. Taiwan); South Korea; Japan (incl. Hokkaido)..... b. var. silvestrii

Figures

Fig. 34 a–f: Boehmeria japonica (L.f.) Miq. var. japonica. a. Habit of stem with female inflorescence-bearing axes; b, c. leaves from different plants; d. leaf, detail of adaxial surface; e. leaf, detail of abaxial surface; f. detail of female inflorescence-bearing axis with crowded flower-clusters (inflorescences). — g–i: Boehmeria japonica var. silvestrii (Pamp.) Friis & Wilmot-Dear. g. Leaf; h. leaf, detail of adaxial surface and large tooth near apex; i. leaf, detail of abaxial surface. — j, k: Intermediate between var. japonica and var. silvestrii. j. Leaf; k. leaf, detail of abaxial surface (a, f: Hicken s.n.; b: Suzuki s.n.; c-e: Fortune s.n.; g-i: Liou 4571; j, k: Furuse 21652; all K). — Scale bars: a-c, g, j = 2 cm; d–f, h, i, k = 2 mm.

Fig. 35 Boehmeria japonica var. tenera (Blume) Friis & Wilmot-Dear. a. Habit of stem with unbranched inflorescence-bearing axes; b. leaf; c. leaf, detail of adaxial surface and margin; d, e. leaf, detail of abaxial surface from different plants; f. detail of stem and stipule; g. detail of male inflorescence-bearing axis with two flower-clusters (inflorescences); h. male bud; i. detail of female inflorescence-bearing axis with two flower-clusters (inflorescences); j, k. fruiting peri- anths from different plants (a, e, f, i: Farges 608; b-d: Henry 4728; g, h: Furuse 42315; j: Silvestri 400; k: Cheng 201; all K). — Scale bars: a = 2 cm; b = 1 cm; c-e = 2 mm; f -k = 1 mm.

Fig. 9 Boehmeria leptostachya Friis & Wilmot-Dear. a. Part of stem with opposite leaf-pairs and bisexual inflorescence-bearing axes; b. detail of leaf margin; c. leaf, detail of adaxial surface; d. detail of abaxial surface; e. stipule; f. detail of inflorescence-bearing axis showing two bisexual flower-clusters (inflores- cences) in flowering stage; g. detail of inflorescence-bearing axis showing one bisexual flower-cluster (inflorescence) in fruiting stage (with pedicels of fallen male flowers); h. male bud; i, j. fruiting perianths (from different plants) (a–d, g: Geesink 7428, K; e, f, h, i: Kerr 16899, K; j: ‘Class of 1958’ 2437, PE). — Scale bars: a = 5 cm; b = 5 mm; c-j = 1 mm.