Japan • Aomori Prefecture, Hiromae City, Mt. Iwaki, 2 Aug 1907, S. Okamura s.n. (holotype: NICH 37785!), (Figs 2, 3).

Plants small, in loose tufts. Stems 5–8 mm, simple, cross-section of stem rounded to oval, diameter 0.11–0.15 mm, central strand present. Leaves homomallous, falcate-secund when dry, erect-spreading when moist. Leaves lanceolate at base, gradually tapering into a channelled acumen; costa excurrent as an awn, mamillose dorsally, in transverse section with differentiated guide cells, with dorsal and ventral epidermis and few substereids on dorsal side of guide cells or poorly differentiated; margins plane or slightly incurved distally, entire or crenulate in distal part of acumen; lamina unistratose, occasionally partially 2-stratose in distal portion, margins 1-stratose; distal and median laminal cells short rectangular to subquadrate, with moderately thickened walls, (6–)8–15(–19) × 4–8 µm; basal juxtacostal cells elongate-rectangular, moderately thick-walled, non-porose, 30–50 × 5–9 µm; alar cells gradually enlarged, not sharply differentiated, unistratose, scarcely inflated, composed of short-rectangular to quadrate inflated cells, non-porose, 24–45 × 14–20 µm.

Autoecious. Perigonia terminal closely located below the perichaetia. Perigonial leaves small, ovate-lanceolate to triangular, 0.65–0.75 × 0.45–0.55 mm, costa present or absent; Perichaetial leaves with sheathing base, abruptly into a channelled acumen. ca. 3.2 mm long. Sporophyte single in perichaetium. Seta straight, 5.5–7.5 mm long, yellowish-brown. Capsules obovate, curved and strumose, smooth when dry; Exothecial cells irregular, short rectangle, thick walled; Calyptra not seen; Operculum not seen; Annulus persistent, one row of small cells; Peristome teeth to 0.35 mm long, orange-brownish below, whitish in distal portion, divided into two prongs to the middle, vertically pitted-striolate below, papillose above. Spores 14–17 µm.

The nomenclatural status of the cited specimen (S. Okamura s.n.; NICH 37785) should be stated first. According to the protologue of Dicranum schistioides (Ihsiba 1929), the type specimen of this species was collected from “津軽富士” (Tsugarufuji; in English), which is another name for Mt. Iwaki (岩木山; in Japanese) in Aomori Prefecture, Japan (Tokuhisa 1978). During this study, we extensively searched the bryological collection of NICH and located only one specimen of D. schistioides, namely “S. Okamura s.n.” (NICH 37785), which was detected by V. F. Brotherus and collected by S. Okamura from “Mt. Iwaki, Mutsu” (Fig. 1). Since Aomori Prefecture was administratively part of “Mutsu” (Sanseidohenshuusho 1975), we thus deemed that this specimen shares the same collection site as recorded in the protologue of D. schistioides (Ihsiba 1929). Furthermore, as the original collection of S. Okamura was supposedly deposited in NICH (Vitt et al. 1985), it can be thus inferred that the present specimen is the holotype of D. schistioides (Art. 9.1; Turland et al. 2018). As Mt. Iwaki (Fig. 1) is now included in Hiromae City, Aomori Prefecture, we provided a corrected type citation above.

In addition, nomenclature of Dicranum schistioides also needs a brief discussion here. This species is nomenclaturally valid although it was only described in Japanese at the time of publication (Art. 39.1; Turland et al. 2018). Later, it was transferred to the genus Arctoa without providing a basionym or replaced synonym (Ihsiba 1932). However, the taxonomic authority of its basionym “(Broth.)” and Japanese name “たかねかもじごけ” were clearly given, which should be regarded as an indirect reference (Arts. 38. 14 & 41.3; Turland et al. 2018), giving A. schistioides a valid taxonomic status.

Arctoa schistioides is distinguished by the following characteristics: (1) a strumose capsule (Fig. 2A, B), (2) a smooth capsule when dry (Fig. 2B); (3) the presence of a central strand (Fig. 3C), (4) gradually enlarged alar cells (Fig. 3A, H), (5) a nearly homogeneous costa structure without stereids (Fig. 3D), (6) mamillose leaf subula (Fig. 3B, D), (7) distal laminal cells that are subquadrate to short rectangular (Fig. 3E), (8) elongate-rectangular basal juxtacostal cells (Fig. 3G), (9) a persistent annulus comprised of small cells (Fig. 2I), (10) perigonia located just below the perichaetia (Fig. 2A) and (11) irregular, short rectangle, thick-walled exothecial cells (Fig. 2J). These characteristics imply that A. schistioides actually belongs to Kiaeria falcata due to the lack of distinct morphological differences between the two species (Newmaster 2007a; Brugués and Ruiz 2012; Lüth 2019).

Figure 2. 

Arctoa schistioides (Broth. ex Ihsiba) Ihsiba A plants B capsules, arrows shows strumose C spores D, E, F peristome teeth G perigonial leaves H perichaetial leaves I annulus J exothecial cells. All from the holotype (S. Okamura s.n. [NICH 37785]).

Figure 3. 

Arctoa schistioides (Broth. ex Ihsiba) Ihsiba A leaves B apex C cross section of stem D cross sections of leaf E upper laminal cells of leaf F middle laminal cells of leaf G basal juxtacostal cells H alar cells. All from the holotype (S. Okamura s.n. [NICH 37785]).

Morphologically, Arctoa schistioides may be confused with A. fulvella (Dicks.) Bruch & Schimp. due to their similar plant morphology and leaf shape (Noguchi 1987; Newmaster 2007a, 2007b; Lüth 2019). However, the alar cells of A. schistioides are gradually enlarged and not sharply differentiated (Fig. 3A, H), whereas those of A. fulvella are clearly delimited and well differentiated (Noguchi 1987; Gao et al. 1999; Ochyra and Buck 2003; Newmaster 2007b; Lüth 2019). Additionally, these two species can be distinguished by their capsule morphology: A. schistioides exhibits smooth capsules when dry, characterised by a distinct strumose (Fig. 2A, B). In contrast, A. fulvella possesses distinctly ribbed capsules when dry and lacks a strumose structure (Noguchi 1987; Ochyra and Buck 2003; Newmaster 2007b; Lüth 2019). Notably, both species share peristome teeth that are divided into two prongs at the mid-point (Fig. 2D; Noguchi (1987); Newmaster (2007b)); however, this division is not always conspicuous in A. fulvella, as the teeth occasionally appear undivided and perforated near the middle (Gao et al. 1999; Ochyra and Buck 2003; Lüth 2019).

Arctoa schistioides is easily confused with A. blyttii (Bruch & Schimp.) Loeske. However, the leaves of A. schistioides are homomallous and falcate-secund when dry (Fig. 2A), while those of A. blyttii are erect-spreading and flexuose (Newmaster 2007a; Brugués and Ruiz 2012). Additionally, the perigonia of A. schistioides are situated just below the perichaetia (Fig. 2A), whereas those of A. blyttii are terminal on a separate branch or positioned far below the perichaetia (Newmaster 2007a; Brugués and Ruiz 2012). Another distinguishing feature is that the exothecial cells of A. schistioides are irregular, short rectangular and thick-walled (Fig. 2J), while those of A. blyttii are rectangular and thin-walled (Brugués and Ruiz 2012). Furthermore, the annulus of A. schistioides is persistent and consists of a single row of small cells (Fig. 2I), whereas the annulus of A. blyttii is deciduous and composed of three rows of large cells (Brugués and Ruiz 2012, 2015).

Arctoa schistioides is also morphologically similar to A. starkei (F. Weber & D. Mohr) Loeske and A. glacialis (Berggr.) Fedosov, Jan Kučera & M. Stech. However, the upper laminal cells of the latter two species are long and rectangular and their capsules are ribbed or grooved when dry (Newmaster 2007a). In contrast, the upper-middle cells of A. schistioides are short rectangular to subquadrate (Fig. 3E) and its capsules are smooth when dry (Fig. 2A, B). Additionally, Kiaeria falcata var. serratifolia Sakurai, a taxon endemic to Japan, can only be distinguished from A. schistioides by its serrate leaf margins (Sakurai 1952), whereas the latter species are smooth or crenulate leaf margins in the distal part of the acumen (Fig. 3A, B).

Dicranum hakkodense Cardot, an intriguing species that shares the Japanese name “タカネカモジゴケ” with Arctoa schistioides, but can be distinguished from the latter species by several characteristics. The leaf tips of D. hakkodense are moderately fragile and the leaves are straight or only slightly falcate-secund when dry (Ignatova and Fedosov 2008; Huang et al. 2023, 2024). In contrast, the leaf tips of A. schistioides are robust, the leaves are homomallous and falcate-secund when dry (Fig. 2A). Furthermore, D. hakkodense exhibits clearly differentiated alar cells, a cross section of the costa that contains distinct stereids and a capsule that lacks strumose features (Ignatova and Fedosov 2008; Huang et al. 2023, 2024). Conversely, A. schistioides possesses alar cells that are not sharply differentiated (Fig. 3A, H), a cross section of the costa that lacks stereids (Fig. 3D) and a capsule that exhibits strumose features (Fig. 2A, B).

In conclusion, we propose Arctoa schistioides as a new synonym of Kiaeria falcata.