Saxicolella nana Engl. (Engler 1926: 456)

Bidault, Ehoarn, Boupoya, Archange, Ikabanga, Davy U., Nguimbit, Igor, Texier, Nicolas, Rutishauser, Rolf, Mesterhazy, Attila & Stevart, Tariq, 2023, Novitates Gabonenses 93: a fresh look at Podostemaceae in Gabon following recent inventories, with a new combination for Ledermanniella nicolasii, Plant Ecology and Evolution 156 (1), pp. 59-84 : 59

Saxicolella nana Engl. (Engler 1926: 456)

Figs 7C , 8H, I

Type.

CAMEROON • Nyong river , south of Yaoundé ["Auf Felsblöcken im Nyong, südlich von Jaunde "]; Jan. 1914; [3°30 ’30” N, 11°29 ’33” E]; 640 m; fl., fr.; Mildbraed 7749a; holotype: B [B100294988]; isotype: U [U1518023] .

Distribution.

Cameroon, Gabon. Before 2018, this species was only known from two collections made at Mbalmayo, Cameroon, in the Nyong river. Johannes Mildbraed collected the first specimen in 1914 (Mildbraed 7749a), and in 2007, a second specimen (Kato et al. CMR-129) was discovered at the type locality. Since 2018, the authors and colleagues collected S. nana nine times in Gabon, where it is now known from the Ogooué, Ivindo, and Okano rivers. It is expected to occur in other rivers in Gabon, as well as in Equatorial Guinea (since it occurs in Cameroon as well as Gabon) and the Republic of the Congo, since it has been collected at its border with Gabon.

Habitat and ecology.

Falls and rapids in rivers from ca 50 to 900 m wide, 160-650 m in elevation. This species is widely distributed, but appears to be rare when encountered. In Gabon it is apparently restricted to micro-habitats submitted to slow-flowing water. It seems to form small, monospecific patches. In Booué, it was found in the vicinity of L. pusilla , and may share the same micro-habitat. In Gabon, flowers and fruits were collected in February, July, and August, in Cameroon, in January and February.

Notes.

Cheek et al. (2022), in their taxonomic monograph of Saxicolella , mention that the recent material collected by the authors of the present study and colleagues most likely represent a new, yet undescribed species that they mention as an unplaced "sp. A", being morphologically close to S. nana (subg. Saxicolella Saxicolella ). The preliminary description provided by Cheek et al. (2022) mentions that it would differ from S. nana by having roots long and ribbon-like with the shoots lacking visible stems, and arising along the margins of the root in rows (vs disc-like, crustose, the shoots with visible stems, arising from the centre of the root in a cluster); leaves entire, linear, and not trifid from a point ca 1.5 mm from the base; the ovary sessile (the staminal filament inserted at its base), not with a distinct gynophore; fruit 8-ribbed (not 6-ribbed). However, as stated by the authors, they did not have access to the Gabonese material, and these observations were made from the associated pictures of living plants made in the field. After close examination of the Gabonese material against the characters mentioned by Cheek et al. (2022), as well as the collection Kato et al. CMR-129 from the type locality of S. nana and widely accepted as belonging to this species, we noted that both the Gabonese and Cameroonian material showed all roots as not crustose, but ribbon-like, 0.5 to 1.5 mm wide. Branching root-ribbons may produce a carpet-like “crust” by creeping over each other. In addition, the drawing 37C by Pohl in Engler (1930: 48), displays a ribbon-like root, ca 1 mm wide, inconsistent with his own description of the species. Unfortunately, Cusset’s description in Flore du Cameroun ( Cusset 1987), solely based on the type material, is not very helpful for this particular character either: "partie basale thalloïde foliacée, profondément divisée’’ is a rather generic statement that could either correspond to crustose root or ribbon-like root. We believe it is best to consider S. nana as having ribbon-like roots that may produce a carpet-like crust by creeping over each other (hence Engler’s probable mistake). Regarding shoots arising along the margins of the root in rows (vs arising from the centre of the root in a cluster), our examination of Kato et al. CMR-129 showed all roots carrying short-shoots along the flanks or margins, not at the centre. Again, the Gabonese material shows a similar feature. Unlike stated by Cheek et al. (2022), Saxicolella nana is described by Cusset (1987) as having stemless or sub-stemmed shoots ("pousses acaules ou subacaules"). The recently collected Gabonese material is also consistent with this description. Leaves of the Gabonese material were mentioned as entire and linear by Cheek et al. (2022) (vs trifid for S. nana ). Close observation of the material revealed the presence of bifid leaves, and observations on Kato et al. CMR-129 showed not only trifid leaves, but also bifid and linear ones. We believe this character can be variable, and a clear overlap suggests it may not be possible to differentiate the Gabonese material from the Cameroonian based on this feature. In addition, the ovary was mentioned as sessile on the Gabonese material by the authors of the monograph, but close observation shows that this material has a distinct gynophore, with the filament not inserted at the base of the ovary. Finally, Cheek et al. (2022) mentioned the Gabonese material as having 8-ribbed fruits, versus 6-ribbed for S. nana . The Gabonese collections indeed show eight ribs, including two commissural, but it is also the case for the collection Kato et al. CMR-129 from the type locality. The original description by Engler (1926) mentioned “6-nervium” in Latin, but Pohl’s drawings ( Engler 1930: 48) show an ovary with six ribs in addition to two commissural ribs that are rather drawn as depressions. In addition, “ribs” on ovaries at anthesis do not appear to be prominent, but rather as darker lines, and commissural ones are indistinct from non-commissural at this stage, whereas, as suggested by Pohl’s drawings, the non-commissural ribs become prominent on the fruit, unlike the commissural two. We believe Engler was referring only to the six non-commissural ribs when writing “6-nervium”. Cusset, in Flore du Cameroun, as well as Kato (2013) state that S. nana has eight ribs. As a consequence, the recently collected Gabonese material is consistent with Cusset’s updated description of S. nana , as having eight ribs (including the commissural ones). For all these reasons, we choose to consider the Gabonese material as a member of this species, and not as a separate, undescribed species.

Figures

Figure 7. Distribution maps of Podostemaceae in Gabon. A. Ledermanniella pusilla (red diamonds), L. tenuifolia (grey circle). B. Ledermanniella pygmaea (grey circles), Macropodiella heteromorpha (red diamonds). C. Macropodiella hallaei (grey circles), Saxicolella nana (red diamonds). D. Tristicha trifaria (grey circles).

Figure 8. Photographs illustrating a selection of species treated in this paper. A, B, C. Ledermanniella schlechteri (A: Texier et al. 2335; B: Texier et al. 2274; C: Texier et al. 2250). D, E. Ledermanniella tenuifolia (D: Boupoya et al. 2417; E: Boupoya et al. 2426). F, G. Macropodiella hallaei (Texier et al. 2249). H, I. Saxicolella nana (Boupoya et al. 1952). Photographs: Nicolas Texier (A-C, F, G), Archange Boupoya (D, E, H, I).