Trichomanes parvulum, Poir. (Poiret, 1808

Dubuisson, Jean-Yves, Nivart, Adele, Senterre, Bruno & Rouhan, Germinal, 2021, New taxonomic treatment for Trichomanes parvulum Poir. and T. dregei Bosch (Hymenophyllaceae, Polypodiidae), Phytotaxa 523 (1), pp. 119-125 : 119-120

Trichomanes parvulum

As highlighted by Dubuisson et al. (2018), Trichomanes parvulum is listed in some taxonomic databases (e.g. Tropicos, https://tropicos.org/name/26603003; African Plant Database 2021), likely based on authoritative taxonomic synopsis (e.g. Roux 2009), as a synonym of Hymenophyllum sibthorpioides (Bory ex Willd., in Willdenow 1810: 498) Mett. ex Kuhn ( Kuhn 1868: 41). But Trichomanes parvulum could also be treated either as a taxonomically accepted species (Kuhn 1968; Christensen 1932; Tardieu-Blot 1960), or as a heterotypic synonym of Crepidomanes minutum ( Blume 1828: 223) K. Iwats. ( Iwatsuki 1985: 524) , as suggested Dubuisson et al. (2018). We may note here that Christensen (1932), even if he keeps the species T. parvulum distinct, suggests the synonymy with H. sibthorpioides . In addition, the Tardieu-blot’s (1960) treatment contradicts a previous publication ( Tardieu-Blot 1951) where she considers T. parvulum as synonym to H. sibthorpioides . This same treatment under H. sibthorpioides is also included in her posthumous publication ( Tardieu-Blot 2008).

All these uncertainties in taxonomic treatments result especially from the fact that Hymenophyllum sibthorpioides and Crepidomanes minutum are tiny filmy ferns that are overall similar and share the same habitat (low to mid-elevation rainforests) in eastern Africa and the western Indian Ocean, and can thus grow in sympatry. This sympatry and their close morphology explain why the two species are often confused in the wild and in collections even sometimes by pteridologists ( Fig. 1A–B ). Because type specimens of T. parvulum are sterile and relatively poorly preserved material ( Fig. 1C–D ), and the protologue of that name is imprecise ( Poiret 1808), it makes so far its morphological characterization difficult and its taxonomic treatment controversial.

In order to clarify the taxonomy of C. minutum and H. sibthorpioides , we aimed at characterizing the puzzling name T. parvulum . Therefore, we studied in detail all plant fragments of the type material of T. parvulum kept in the P herbarium (L.-M.A. du Petit-Thouars s.n., P00065014, P00065015; herbarium acronyms follow Thiers 2021).

For this purpose, we first summarised the main characters that distinguish the two species C. minutum and H. sibthorpioides ( Tab. 1 ). Considering fertile specimens, the distinction is easy: C. minutum has tubular to campanulate sori typical of trichomanoids (to which the species belongs) while H. sibthorpioides has bilabiate sori typical of its genus; additionally, sori margins are toothed in H. sibthorpioides vs. non-toothed in C. minutum . Considering sterile specimens, rhizome characters are sometimes informative: in Crepidomanes (C.Presl) C.Presl (Presl 1849: 258) rhizomes are rootless and densely covered with short dark brown hairs that may extend to the base of the stipes whereas in Hymenophyllum Sm. ( Smith 1793: 418) rhizomes bear sparse reddish-brown hairs (or may appear glabrous if aged) with sparse fine roots. The entirely sterile type material of T. parvulum comprises a single rhizome fragment that is connected to a frond ( Fig. 1C , green arrow). This rhizome is glabrous, hence suggesting to ascribe it to H. sibthorpioides . By studying the frond lamina, two morphotaxa can nevertheless be clearly distinguished: one with the clear greenish-grey coloured dry fronds that show cellular characters close to those of C. minutum ( Fig. 1C–D , clear fronds without arrows; Fig. 1F ), and one with the dark reddish-brown coloured dry fronds that show cellular characters close to those of H. sibthorpioides ( Fig. 1C , blue arrows; Fig. 1G ). In addition, one clear frond appears to have proliferation ( Fig. 1D , red arrow). Looking at this latter character in detail ( Fig. 1E , red arrows), such proliferation is double and originates on the rightmost stipe. The proliferation that defines the budding of a new frond on a stipe is an exclusive autapomorphy of C. minutum ( Ebihara et al. 2006) . Concerning segment apices, the ends of the segments of almost all the fronds are damaged, but for at least the frond with proliferation (Fig. D, red arrow), some preserved segments show emarginate apex as expected for C. minutum .

The characters of rhizome, stipe and lamina of the type of T. parvulum therefore clearly indicate that the different fragments represent a heterogeneous mixture of the two species H. sibthorpioides and C. minutum . Consequently, a lectotype corresponding most nearly with the original description or diagnosis must be designated (Art. 14, Turland et al. 2018). The original description of T. parvulum describes the sorus as urceolate, or as a small, dilated cup at its extremity. The adjective urceolate describes a campanulate form with the median part slightly broader than the mouth. This depicts rather a trichomanoid sorus than a Hymenophyllum sorus, and the dilated lips are also characteristic of C. minutum .

Consequently, the description of the sori and the presence of C. minutum in the type material of T. parvulum led us to designate as lectotype of T. parvulum the fragments identified as C. minutum . This is also in agreement with the illustration of T. parvulum by Hooker (1844: Tab. 39 A) which clearly shows a C. minutum with typical campanulate sori. As the earlier name T. parvulum has priority over Trichomanes minutum Blume (1828: 64) , it is the correct name to apply, and it must be combined under Crepidomanes .

Figures

FIGURE 1. Comparison of Cr. minutum, H. sibthorpioides, and the holotype of T. parvulum. A) Cr. minutum in situ, B) H. sibthorpioides in situ; C) and D), type material of T. parvulum P00065014 (C) and P00065015 (D) (blue arrows: reddish-brown fronds corresponding to fronds of H. sibthorpioides, now kept separate under P01190780 and P01190781; green arrow:hairless rhizome; red arrow:proliferations); E) Double proliferation on a frond of the type P00065015; F) greenish-grey frond cells; G) reddish-brown frond cells. C and D, scale = 1 cm; F and G, scale = 100 µm.