Anthoceros subtilis Stephani (1916: 1003)

Ibarra-Morales, Ariadna, 2021, Anthoceros subtilis and A. telaganus (Anthocerotaceae): two new records for America from the hornwort flora of Mexico, Phytotaxa 529 (1), pp. 93-104 : 94-97

Anthoceros subtilis Stephani (1916: 1003) . Fig. 2 , 3A–B .

Type :— INDIA. South Canara, Mangalore, August 1907, R. Pfleiderer 6127 (holotype G-G00060882) .

For synonyms see Hasegawa (1998), Peng & Zhu (2013) and Villarreal et al. (2015).

Monoicous. Plants growing on soil in rosettes. Thallus light to dark green, up to 2.6 mm in length and 3.5 mm in width. Dorsal lamellae absent or scarce. Margins of thallus highly crenulate. Thallus up to 22 cells thick in solid regions (106– 599 μm), cavernous, schizogenous cavities 75–150 × 115–155 μm, 1–3 layers of schizogenous cavities in transverse section. Single chloroplast per cell with central pyrenoid. Rhizoids hyaline to brown; unbranched. Ventral spheric Nostoc colonies abundant. Antheridia scattered. Antheridial chamber with up to 15 antheridia per cavity, antheridia yellowish to dark brown, 102–125 μm μm in length, with four-tiered jacket cell arrangement. Archegonia scattered, with five neck cells covered by mucilage. Involucres erect, cylindrical, up to 1.5 mm long, with smooth surface or with very few lamellae, 3–6 cells thick in solid regions (28–159 μm), cavernous, schizogenous cavities 15–30 × 22–47 μm, one layer of schizogenous cavities in transverse section.

Up to three sporophytes per plant. Sporophyte foot with a layer of palisade-like cells. Capsules erect, up to 10 mm long, opening by two longitudinal valves. Epidermal layer of capsule one cell thick in transverse section. Epidermal cells of capsule rectangular, elongated, 7.5–19 × 37–85 μm and thick-walled, stomata abundant (17 per mm 2), stomata 24–40 × 42–58 μm. Assimilative layer 4–5 cells (26.4–95.8 μm) thick in transverse section. Columella present within the sporophyte, extending to the tip, rigid and brown, 16 cells in cross section. Sporogenous layer one spore tetrad thick. Pseudoelaters short, thin-walled, light brown to dark brown at maturity, 42.8–81.6 μm long (65.1±26.9, n=22), 3.9–33.4 μm in width, and 1–5 cells at maturity. Spores light brown to brown at maturity, 26.6–42.1 μm (34.9±3.5, n=31) in equatorial diameter. Distal spore surface with numerous papillate to tuberculate projections, up to 3 μm in height and 2.2 μm in width, sometimes confluent at their bases. Proximal spore surface with a distinct trilete mark, 0.89–1.5 μm in width, irregularly covered with verrucae; triangular areas covered with numerous papillate to tuberculate projections, up to 2.8 μm in height and 2.5 μm in width, sometimes confluent at their bases. The entire spore surface is covered with minute papillae, up to 0.8 μm in height and 0.4 μm in width.

Habitat: —Growing on the sides of roads and around coffee plantations in tropical montane cloud forest, with other hornworts such as A. telaganus Stephani (1916:1005) and Phaeoceros sp. , ca. 1300 m.

Distribution:— China, Central African Republic, India, Japan, Korean Peninsula, Nepal, Thailand, Vietnam ( Asthana & Srivastava 1991, Hasegawa 1998, Peng & Zhu 2013, Villarreal et al. 2015, Choi et al. 2021) and Mexico.

Affinities and differentiation:—Mexican populations of A. subtilis are characterized by small, brown spores (26.6–42.1 μm), with papillate to tuberculate projections covering both proximal and distal surfaces. Within Mexican taxa, A. aff sambesianus Stephani (1916: 996) shares with A. subtilis the small sized plants ( Ibarra-Morales et al. 2015), up to 7 mm in diameter in A. aff sambesianus and up to 2.6 mm in length and 3.5 mm in width in A. subtilis , but can be differentiated by spore ornamentation pattern. Anthoceros aff sambesianus possess a low reticulated network with foramens and central sub-globose tubercles in triangular areas of proximal face ( Fig. 3 C-D), while A. subtilis presents irregular tuberculate projections and minute papillae ( Fig. 3 A-B).

According to Peng & Zhu (2013), A. subtilis resembles A. angustus Stephani (1916: 1001) in spore morphology, nevertheless, they can be differentiated by the presence of marginal spongy gemmae on the thallus margins of A. angustus , absent in A. subtilis ; and by the size of the pseudoelaters, which are considerably longer in A. angustus (40–496 µm) and somewhat shorter in A. subtilis (42–199 µm). Anthoceros angustus is restricted to Asia ( Peng & Zhu 2013).

Specimens examined:— MEXICO. Chiapas: Unión Juarez , roadside, 1438 m, 15°04’03” N 92°04’53” W, 26 July 2019, Ariadna Ibarra-Morales 2019 -14b ( FCME) .

Remarks: —Mexican populations of this species were always found growing with other bryophytes. In one of the most crowded patches, some rosettes of A. subtilis were formed by very small plants of less than 1 mm in capsule height and thallus diameter. Some also exhibited an unusual erect morphology of the thallus, similar to the thallus morphology that has been reported in A. erectus Kashyap (1915: 9) ( Asthana & Srivastava 1991). In these dwarf plants, the surface of the thallus was barely large enough for the base of the involucre to fit. This dwarf morphotype was probably a result of space and resource availability and may have also been affected by a fungal infection, since red hyphae were found within the gametophyte of some of these plants.

Previous reports of this species from Asia ( Asthana & Srivastava 1991, Hasegawa, 1998; Peng & Zhu 2013) document slightly larger thallus size (5–6 mm in diameter) and larger spores (35–45 µm) than those of Mexican plants (thallus up to 3.5 mm and spores of 26.6–42.1 μm), but spore morphology is very similar to that illustrated by Peng & Zhu (2013) from China.

Figures

FIGURE 2. Anthoceros subtilis. A. Whole plants. B. Antheridia with tiered jacket cell arrangement. C. Thallus transverse section with schizogenous (Sc) cavities and Nostoc colonies (Nos).D. Dorsal surface cells with chloroplasts displaying central pyrenoids. E. Sporophyte transverse section with epidermal layer (ep), assimilative layer (as) and spores (sp). F. Epidermal layer of capsule with stomata. G. Spore ornamentation on distal surface (i), proximal surface (ii) and pseudoelaters.

FIGURE 3. Scanning electron microscopy (SEM) micrographs of spores of Anthoceros subtilis and A. aff sambesianus. A–B. Anthoceros subtilis (from A. Ibarra-Morales 2019-14b) C-D. Anthoceros aff sambesianus (from A. Ibarra-Morales 19b). A & C distal surface, B & D proximal surface.