GABON • Booué; 29 Jul. 1966; [0°05’24”S, 11°55’59”E]; 165 m; fl., fr.; N. Hallé & A. Le Thomas 206; holotype: P [P00179254].

Gabon, Republic of the Congo. Before 2017, this species was only known from two collections: the type collected in 1966 at Booué in the Ogooué river in Gabon, and a collection made by Sita at the Mandoro rapids in the Republic of the Congo, in a tributary of the Kouilou river. Between 2018 and 2020, six collections were made, and its presence was confirmed at Booué. It was also discovered at another site in the Ogooué, the Poubara falls, and it is therefore probably present elsewhere on the Ogooué river, though it has not yet been collected at several other visited sites along the river. One collection is also known from the Okano river, a tributary of the Ogooué. The collection by Sita was determined by Colette Cusset in 1998, but since the P (P00179255) specimen shows stems branched multiple times, it was mentioned as doubtful in the synoptic revision of the genus (Cheek et al. 2017), though not providing a different determination since they did not observe the specimen. Cheek et al. (2017) also described Inversodicraea tchoutoi Cheek, to which they associate the collection Letouzey 10299, initially determined as I. annithomae by Cusset. Cheek et al. (2017) hence consider I. annithomae as a species endemic to the Booué rapids. Since recently collected material revealed morphological variability in I. annithomae (see Notes), we chose to stick with Cusset’s determination of the Sita collection, and to consider this species as present in the Republic of the Congo. Moreover, since this species was found at Poubara (only ca 150 km away from Sita’s collection site), it is also probably present in the Ogooué river upstream of Poubara, where rapids are present only ca 100 km away from the Sita collection site.

Figure 3. 

Photographs illustrating a selection of species treated in this paper. A, B. Inversodicraea annithomae (A: Boupoya et al. 2444; B: Boupoya et al. 2689). C. Inversodicraea boumiensis (Boupoya et al. 1748). D, E. Inversodicraea nicolasii (D: Texier et al. 2247; E: Texier et al. 2334). F, G. Inversodicraea paulsitae (Boupoya et al. 1750). H, I. Inversodicraea tenax (H: Boupoya et al. 2432; I: Boupoya et al. 2430). Photographs: Igor Nguimbit (A), Archange Boupoya (B, C, F–I), Nicolas Texier (D, E).

Rapids and falls in rivers from ca 90 to 410 m wide, 160–400 m in elevation. It appears to be quite rare at the sites where it has been encountered. Flowers and fruits were collected in July and August, and fruits in September. It is mostly found in micro-habitats submitted to strong currents, but can also be found in slower-running water.

Recent collections revealed a strong morphological variability: the collection Boupoya et al. 2698 shows cable-like horizontal stems more than 5 mm thick and up to 30 cm long, sparsely covered by scale-leaves, and bearing axillary, short flowering shoots abundantly covered by scale-leaves, while the collection Boupoya et al. 2684 shows upright flowering shoots of up to 2 cm long, with stems up to 1.5 mm in diameter, abundantly covered by scale-leaves, and attached to a ribbon-like root. Intermediate forms were also collected (Boupoya et al. 2672 and 2682), showing typical features of I. annithomae, but with upright to horizontal shoots up to 5 cm long and 3 mm thick, still covered by scale-leaves on ca 50–60% of their surface. The three forms (small and upright, medium and upright to horizontal, large and horizontal) may be developmental stages as well as environmental modifications, but more precise field observations and habitat characterization is needed to explain this morphological variability. The morphological variability of I. annithomae was previously indirectly postulated by Cusset, who determined the sterile specimen Letouzey 10299 as I. annithomae, probably considering that the absence of long, flexible flowering stems (as for the typical form of the species) was the result of a juvenile stage (Cheek et al. 2017).

Figure 4. 

Distribution maps of Podostemaceae in Gabon. A. Inversodicraea annithomae (grey circles), I. boumiensis (red diamonds). B. Inversodicraea nicolasii (red diamonds), I. paulsitae (grey circles), I. tenax (orange triangles). C. Inversodicraea thollonii (red diamonds), Ledermanniella bifurcata (grey circles). D. Ledermanniella aloides (grey circles), L. schlechteri (red diamonds).

GABON • Boumi waterfalls at Mbigou [“Chutes de la Boumi à Mbigou”]; 5 Jul. 1927; [1°54’16”S, 11°54’31”E]; 665 m; fl., fr.; Le Testu 6536; holotype: P [P00179270]; isotypes: BM [BM000910485], BR [BR0000006267230], P [P00179271].

Endemic to the Louétsi river, south-eastern Gabon. Before 2017, this species was only known from a single collection made at Boumi falls, near Mbigou. In 2018, it was discovered at two additional sites on the same river, at Issala and Dibwangui.

Rapids and falls in rivers from ca 30 to 50 m wide, 380–665 m in elevation. It appears to be very rare at the sites where it has been encountered. Flowers and fruits were collected in July and August. At Dibwangui, it grows intermingled with I. aff. annithomae, in strong currents.

Inversodicraea boumiensis can be distinguished from I. annithomae mainly by its much smaller stature (stem < 1 mm thick and up to 2–3 cm long vs stem up to 3 mm thick and 20 cm long), stem scales (thin and spreading in I. boumiensis vs thick and appressed in I. annithomae), and pollen as dyads (vs monads in I. annithomae). The collection Letouzey 15339 from the Ntem river (Cameroon) was attached to I. boumiensis by Cusset, but later determined as I. tchoutoi (Cheek et al. 2017). After examination of this collection, we choose to follow Cheek et al. (2017).

CAMEROON • Makaka Station [“Makaka, gr. Steine & Felsen in reissendem Bach im gebirgigen Walde”]; 21 Nov. 1908; [4°55’00”N, 9°57’00”E]; 500 m; fl.; Ledermann 1173; holotype: B; isotypes: BM [BM000910414], U [U0005650].

Central African Republic, Cameroon, Equatorial Guinea, Gabon, Angola. This species is known from Gabon by only one collection, N. Hallé 4451, collected in 1968 at the Kinguélé waterfall on the Mbé (Mbei) river in Monts de Cristal. The species is known from at least 15 other collections elsewhere, mainly in Cameroon (12 collections).

Rapids and falls in rivers from ca 25 to 180 m wide, 5–1500 m in elevation. No information could be found regarding its abundance on sites where it has been found. Flowers and fruits were collected in January, February, and March (in Cameroon and Gabon), in August (in Angola), and in November (Cameroon). Its sole collection from Gabon also contained a few individuals of I. tenax, indicating that both species probably grow intermingled at this site (Kinguélé waterfalls). Ledermanniella bifurcata (Engl.) C.Cusset and L. letestui were also collected at this site, but no information is provided on the habitat partitioning.

According to Cheek et al. (2017), Cusset had a morphologically broad concept of the species. Recent examination by Cheek et al. (2017) of material associated to this species by Cusset revealed that some collections belong to other species, such as Inversodicraea digitata H.E.Hess or I. tchoutoi. Cheek et al. (2017) consider that this species may even be restricted to Cameroon, with records from Equatorial Guinea, Angola, C.A.R., and Gabon representing other species, but they did not examine all collections and did not provide new identifications for the non-Cameroonian collections. Examination of the Gabonese material in Paris did not reveal significant differences from the original description. Hence, we choose to consider the collection N. Hallé 4451 as belonging to I. cristata. Recent inventories were carried out at the Kinguélé waterfall by the authors and colleagues and revealed that the site is now completely devoid of Podostemaceae. The construction of two hydroelectric dams between 1969 and 1978 deeply modified the water availability at the site. Inversodicraea cristata was not collected elsewhere in Gabon.

GABON • Ogoulou rapids at Mitingo [“Haute Ngounyé, rapides de l’Ogoulou à Mitingo”]; 19 Jul. 1926; [1°55’00”S, 11°18’00”E]; 200 m; fl., fr.; Le Testu 6017; holotype: P [P00179287]; isotype: BM [BM000910486].

Gabon, Republic of the Congo. This species is solely known from five collections. Three are known from the Foulakari waterfalls (a tributary of the Congo river) and one from waterfalls on the Nyanga river in Mouvoungou, all in the Republic of the Congo. The only Gabonese collection, Le Testu 6017, was made on the Ogoulou river, a tributary of the Ngounié river, in 1926.

Rapids and falls in rivers from ca 60 to 190 m wide, 130–200 m in elevation. No information could be found regarding its abundance on sites where it has been found. Flowers were collected in May and July, fruits in July, August, September, and October. Another species is found at the Foulakari waterfalls (Ledermanniella bifurcata), though the precise habitat partitioning at this site is unknown.

The type locality (in Gabon) has not been visited since 1926, nor other sites on the Ogoulou river. Sampling on the Ngounié river by authors and colleagues resumed only in 2019 with the exploration of the Impératrice waterfalls in Fougamou, far downstream, and did not lead to the collection of this species.

GABON • Ogooué river at Booué [“Lit de l’Ogooué à Booué”]; 29 Jul. 1966; [0°05’24”S 11°55’59”E]; 165 m; fl., fr.; N. Hallé & A. Le Thomas 205; holotype: P [P00179341]; isotypes: P [P00179342, P00179343].

Endemic to the Ogooué and Ivindo rivers, in Gabon. Before 2017, this species was only known from a single collection made at Booué rapids on the Ogooué. In 2018, it was discovered on the Kongou falls, on the Ivindo river. In 2019, it was discovered on the Touné falls on the Ivindo river, less than 4 km upstream from the confluence with the Ogooué river, 30 km east of the type locality at Booué, and ca 60 km south-west of the Kongou falls. Recent efforts of Podostemaceae inventory at Booué rapids (west of the city) allowed for the rediscovery of this species at its type locality. Nevertheless, it seems very rare at this site, since only one of the 70 collections made at this site between 2019 and 2021 represents this species. Given the considerable size of the rapids area, which is not yet fully explored, it is possibly more abundant than suggested by our data. The type material collected in 1966 includes a lot of individuals, suggesting that the species was, at least then, abundant on site. A natural, partial replacement of this population by another species cannot be excluded.

Figure 5. 

Drawing of Inversodicraea nicolasii. A. Sterile shoot tip with young leaves and stem scales (scale bar: 0.5 mm). B. Fertile shoot before anthesis (scale bar: 5 mm). C. Capsule, one valve removed (scale bar: 1 mm). D. Flower at anthesis (scale bar: 2 mm), ovary with two stigmas dropped. E. Cross-section of the ovary (scale bar: 0.5 mm). F. Detail of the ovary (scale bar: 1 mm). G. Fertile lateral shoot and portion of the main stem (scale bar: 1 cm). A, E, F: Texier et al. 2247; B: Nguimbit et al. 8; C, D: Boupoya et al. 2445; G: Nguimbit et al. 7. Drawings by Anne-Hélène Paradis, based on preliminary drawings by Rolf Rutishauser.

Rapids and falls in large rivers from ca 100 to 500 m wide, 165–425 m in elevation. It appears to be quite abundant at Kongou falls, but rare at Booué and Touné falls. Flowers and fruits were collected in February, July, and August. The species grows intermingled with Ledermanniella schlechteri and Macropodiella hallaei at the Kongou falls. It appears to grow with M. hallaei, M. heteromorpha, and L. pusilla at the Touné falls. It was collected in micro-habitats submitted to strong currents. It is apparently consumed as a vegetable by local people around the Kongou falls in Gabon (boiled in water). Recent footage taken from the Kongou falls also showed an elephant potentially feeding on Podostemaceae, even though it is unclear if this species in particular is targeted.

The recent material collected at Kongou falls on the Ivindo river revealed the presence of few, small but conspicuous triangular to wart-like, patent stem scales, most of them with a distal black dot, mostly grouped below the inflorescences, and very scarce along the stem. Closer examination of the type collection revealed similar stem-scales that were overlooked by Cusset, who initially placed this species in subgenus Ledermanniella rather than subgenus Phyllosoma. Because of the presence of clear stem-scales, we consider that this taxon belongs to the genus Inversodicraea. The recent collections from the Ivindo river show minor differences with the type collection, such as shorter and wider leaves and longer flowers and capsule stalks (up to 3 cm long vs up to 2 cm long), ca 60% of them bearing 3 stamens (vs 2 stamens), but they are otherwise very similar. We do not consider those differences as sufficient to distinguish another taxon. Inversodicraea nicolasii is solely known from four collections, which suggests a morphological variability that was unsuspected until now, and that remains largely undocumented.

GABON • Litsila rapids on the Ngounié river, at Moupoundi [“Moupoundi, rapide Litsila de la Ngounye”]; 20 Jun. 1927; [2°25’00”S, 11°50’00”E]; 250 m; fl., fr.; Le Testu 6510; holotype: P [P00179346]; isotype: BR [BR0000006265069].

Gabon, Republic of the Congo. This species is only known from southern Gabon in the Ngounié and Louétsi rivers, and the Bibaka and Bouenza rivers in the Republic of the Congo.

Rapids and falls in large rivers from ca 50 to 80 m wide, 150–580 m in elevation. It appears to be quite abundant at the few sites where it has been encountered. Flowers and fruits were collected in January, June, and August. Grows intermingled with Inversodicraea annithomae and I. boumiensis in the Louetsi river. This species was solely found in micro-habitats submitted to strong currents.

Before 2017, this species was only known from two collections in Gabon (in 1927 and 1933), and two in the Republic of the Congo (in 1965 and 1975). One of the Gabonese collections was made at the Byoko falls on the “Louétoie” river, that presumably corresponds to the Bongolo falls on the Louetsi river, near Lébamba, a site that was recently explored by authors and colleagues, but without any success. A small hydroelectric facility was built downstream of these rapids, which might have caused the disappearance of this species’ subpopulation. Another species was nevertheless collected at this site, suggesting that I. paulsitae may be less resilient to disturbances and may have been replaced. This hypothesis remains speculative until a complete inventory of these rapids is undertaken. Recent inventories made in the Ngounié river did not collect this species, but the exploration of this major Gabonese river remains largely fragmentary.

ZAMBIA • Victoria Falls, Livingstone Island; [17°55’26”S, 25°51’14”E]; 885 m; fl., fr.; 17 Sep. 1906; Kolbe 3149; holotype: K [K000435188]; isotypes: BM [BM000797689], BOL [BOL135706].

Gabon, Democratic Republic of the Congo, Angola, Namibia, Tanzania, Botswana, Zambia, Zimbabwe. Though recent collections revealed the presence of this species in Gabon, it has in fact been collected in the country as early as in 1926 (Le Testu 5983), but it was misidentified as I. ledermannii by Cusset (see Notes under that species). In addition, examination of the collection N. Hallé 4451 collected on the Kinguélé waterfall on the Mbé river (Monts de Cristal) in 1968, and identified as I. cristata by Cusset, revealed the presence of one fertile individual identified by us as I. tenax, now as the collection number N. Hallé 4451B, separated from N. Hallé 4451A, which represents I. cristata. Material associated with I. ledermannii from the Democratic Republic of the Congo and Angola was not examined for this study, but should be checked for their pollen. If monads are observed (i.e. not dyads as typical for I. ledermannii), this material would be best associated to I. tenax, which could link this species’ original distribution with the newly discovered Gabonese subpopulations.

In Gabon, rapids and falls in rivers from ca 10 to 40 m wide, 50–510 m in elevation (up to 1,700 m in Tanzania). It appears to be quite rare at the few sites where it has been encountered. In Gabon, flowers and fruits were collected in January, June, July, and August. It grows intermingled with I. cristata (N. Hallé 4451A and 4451B) and close to L. pusilla in the Monts de Cristal area. Collections made from small rivers in Monts de Cristal (north-western part of Gabon), as well as larger rivers (Komo, Abanga, Offooué) revealed important ecological tolerance, previously unsuspected for this species.

The discovery of I. tenax in Gabon is surprising, but the pollen as monads shown by the Gabonese material and the close examination of material from southern Africa leaves no doubt about the identity of the Gabonese collections. The recently collected Gabonese material revealed important morphological variability: stem-scales appear to have a variable number of dorsal projections (from 0 to 2, often on the same individuals) and of lobes (3 to 5), whereas Cusset’s description of the species (Cusset 1983) mentioned 3-dentate stem-scales (or with 3 lobes), and most often with 2 dorsal appendages. The Gabonese material also shows a great variety in the proportion of dorsally appendaged stem-scales versus dorsally smooth stem-scales, with some individuals nearly or completely devoid of dorsal appendages. Nevertheless, the pollen as monads consistently observed on this material allows to rule out I. ledermannii (that has pollen as dyads). The examination of the Paris isotype of the recently described I. tanzaniensis (Schlieben 1131A) showed a similar pattern of stem-scales variation, bearing 3 to 5 lobes and from 0 to 2 dorsal appendages, despite the original description (Cheek et al. 2020) mentioning the clear absence of dorsal appendages as a strong difference with I. tenax. Stem-scale variation appears more important at the tip of shoots, on stem-scales subtending spathellae, in both the Gabonese collections as well as I. tenax material and the isotype of I. tanzaniensis. Stem-scale morphological variability within the genus Inversodicraea (including in I. tenax) was already described by Cusset (1983), who mentioned differences in shape and size of stem-scales depending on their position in the stem. Considering the clear continuum between the presence and absence of dorsal appendages, we consider that their absence cannot be used to discriminate material from an otherwise similar species. In addition, all dried capsules observed on the isotype of I. tanzaniensis as well as the Gabonese material showed 8-ribbed capsules, including two commissural ribs clearly visible, whereas Cheek et al. (2020) mentioned I. tanzaniensis as having 6 ribs instead of 8 being a strong difference with I. tenax. Among other differences between I. tenax and I. tanzaniensis are the lengths of the tepals and gynophore. The Gabonese collections showed tepals of ca 0.8 mm of length, similar to what was described for I. tanzaniensis, but slightly longer than mentioned on the original description of I. tenax (0.5 mm), and a gynophore of ca 0.5 mm, similar to what was described for I. tenax, but slightly longer than mentioned for I. tanzaniensis. We believe those differences are not relevant, especially since some variability has also been observed by the authors on other species. Finally, another important character mentioned by Cheek et al. (2020) to discriminate I. tanzaniensis from I. tenax is the length of stem, of up to 20 cm for I. tenax whereas I. tanzaniensis has 5–8 cm long stems. The recently collected Gabonese material showed both stemless individuals (Boupoya et al. 2418, 2420, 2422, 2423), or with stems of up to 3 cm long (Boupoya 1709, 2429, 2430, 2432). We suggest a cautious approach when considering the length of stem as a character to discriminate species. If some Podostemaceae species clearly never show elongated stems, especially in Ledermanniella (L. thalloidea (Engl.) C.Cusset and L. aloides, for instance), the prolific material collected by authors and colleagues suggests a strong intraspecific variability in that matter, that was only sporadically mentioned by Cusset (1983) in Ledermanniella (L. pusilla and L. bifurcata), and that was largely unsuspected for Inversodicraea until now. Recent material collected in Gabon shows that I. annithomae, I. thollonii, and I. tenax (among others) can show stemless shoots as well as developed stems, sometimes on single individuals. In addition, such variability is not surprising considering species that are widespread and known from a variety of micro-habitats, such as I. tenax. When developed stems are observed for a given species, stem length might therefore better reflect ecological preferences. Our observations also suggest that developed stems are often associated with strong currents. For all the above-mentioned reasons, we believe it is best to consider I. tenax as a widespread, morphologically variable (with continuums) and ecologically ubiquitous species, which comprises I. tanzaniensis.

GABON • Lopé rapids in the Ogooué [“Ogooué, rapides de Lopé”]; Jul. 1887; [0°05’38”S, 11°35’24”E]; 115 m; fl., fr.; Thollon 825; holotype: P [P00179381]; isotype: BM [BM000910405].

Endemic to the Ogooué river and its tributaries, in Gabon. Before 2007, this species was only known from two collections, one from the Booué rapids on the Ogooué (N. Hallé 202b, collected in 1966) and one from the rapids of the Ogooué at Lopé (Thollon 825, collected in 1887). In 2007, three sterile collections were made in the Lenké (or Langke) river near Booué (Kato et al. GB11, GB12, and GB13, as mentioned in Koi et al. 2012) that were not identified as I. thollonii until this study. Since 2019, it has been collected 35 times in the Ogooué, and in various tributaries: the Lopé, Létili, Okano, Bissoubilam, Offooué, and Lenké rivers. All of these collections were made less than 2 km upstream from the confluence with the Ogooué river, except for the collection from the Bissoubilam river (ca 6 km away). Most (38 of the 40) of the collections were made in or along a ca 130 km long portion of the Ogooué between Alembé and Makokou in central Gabon, with the exception of two collections made in the Létili river at the border between Gabon and the Republic of the Congo. Those collections are separated from the easternmost others by ca 300 km, and it is expected that I. thollonii is also present in between.

Figure 6. 

Photographs illustrating a selection of species treated in this paper. A, B. Inversodicraea thollonii (A: Boupoya et al. 1948; B: Boupoya et al. 1943). C. Ledermanniella aloides (Boupoya et al. 1749). D, E. Ledermanniella bifurcata (D: Boupoya et al. 1954; E: Boupoya et al. 1958). F, G. Ledermanniella pusilla (F: Boupoya et al. 1733; G: Boupoya et al. 1735). H. Ledermanniella pygmaea (H: Boupoya et al. 1764). Photographs: Archange Boupoya.

Rapids in rivers from ca 20 to 600 m wide, 45–500 m in elevation. It appears to be abundant where encountered. Flowers and fruits were collected in July, August, and September. It was collected with or near to Ledermanniella aloides, L. pusilla, Macropodiella hallaei, and Tristicha trifaria. Collections made from small Ogooué tributaries (Lopé, Bissoubilam, and Lenké rivers), larger tributaries (Okano, Létili, and Offooué rivers) as well as the Ogooué river itself revealed important ecological tolerance, previously unsuspected for this species. It has been noticed in strong as well as slow currents. It has surprisingly not been found on the Ivindo river, even at the Touné falls, close to its confluence with the Ogooué. However, only one day was dedicated to the exploration of this site, and this species may have been overlooked.

As for other species in this study, the newly collected material revealed great morphological variability. Inversodicraea thollonii is among the Podostemaceae species showing both stemmed and stemless shoots, whereas it was originally described as having short stems 1–2 cm long (Cheek et al. 2017). If both forms were usually collected separately, a few recent collections showed stemmed and stemless shoots on single individuals (Boupoya 1948 and 1968, for instance). Stem-scales also show variability, most of them being narrowly triangular with rounded apex as described by Cusset (1983), but in some cases almost linear (Boupoya 2452). Stem-scales are usually spreading and quite delicate (thin). On stemmed individuals, stem-scales are scarce, as described by Cusset (1983), and never covering more than 50% of the stem as the key to all species provided in Cheek et al. (2017) mentions. On stemless individuals, stem-scales are forming a dense crown below the spathellae. With 40 collections to date, I. thollonii is now considered as one of the most common Podostemaceae species in Gabon, even though it is yet to be found in other watersheds than that of the Ogooué river.

CAMEROON • Adamaoua, Pass Tchape; 27 Feb. 1909; [7°23’00”N, 11°55’00”E]; 1500 m; fl.; Ledermann 2785; holotype: B; isotype: BM [BM000910397].

Sierra Leone, Nigeria, Cameroon, Central African Republic, Gabon, Angola. This species was first collected in Gabon in 1993 by Lee White at Lopé, probably not in the Ogooué river, but in a small tributary. We have not been able to retrieve this collection and confirm the identification made by Sosef in 2017. In 2007, another collection, Kato et al. GB-10, was made in the Lenké river near Booué (erroneously labelled as Ledermanniella bifurcata by Koi et al. 2012, see Notes). Since 2017, it has been collected 47 times by the authors and colleagues, including 13 collections from the Louétsi river, 11 from the Komo river, and 23 from the Ogooué or its tributaries. Ledermanniella aloides is the second most commonly found Podostemaceae species in Gabon.

In Gabon, rapids in rivers from ca 10 to 450 m wide, 85–550 m in elevation (up to 1,500 m in Cameroon). It appears to be very abundant where encountered, forming dense, monospecific mats. In Gabon, flowers and fruits were collected in March, July, August, and September, elsewhere also in May, November, and December. In Gabon, it appears to share the same micro-habitat as I. thollonii, L. pusilla, Tristicha trifaria, and was once found near Saxicolella nana. Ledermanniella aloides is often found in relatively slow-running currents.

Ledermanniella aloides, unlike I. thollonii or L. pusilla, never shows developed stems. Nevertheless, some characters used by Cusset (1984) to distinguish it from morphologically close species seem to be more variable than suspected. We observed that the distichous arrangement of leaves and the absence of ramification of the blade are constant throughout the recently collected material, but the presence and shape of stipules, the presence and size of the sheath, and the length and overall shape of the blade can vary. We have noticed three morphogroups occurring in Gabon: one characterized by having all leaves of reproductive rosettes without stipules, with sheaths enlarged but folded and imbricate, and with ensiform (laterally flattened) leaf blade; another group characterized by most leaves with 1–2 stipules at base, with sheaths enlarged but less folded, and leaf blades less obviously ensiform; and a third group with enlarged sheaths bearing obscure (rounded) stipules, and with leaf blade widening on their apical portion, most of them showing a conspicuous midrib. Nevertheless, we are reluctant to name and describe these morphogroups as separate entities, as all characters mentioned here show continua, including in collections from West Africa. Morphological continua between these character states can sometimes even be observed on single shoots, with strong polymorphism of leaves depending on their position relatively to the spathellae. Most of the Gabonese collections studied could be assigned to either one of the three morphogroups based on the observation of the numerous individual shoots they each comprise (usually several dozens or even hundreds of shoots), but all of them also comprise a significant portion of shoots displaying intermediate character states. We believe it is best for now to consider L. aloides as a morphologically variable species, occurring from Sierra Leone to Central African Republic and to Angola. Ledermanniella aloides remains recognizable from any other Ledermanniella species mainly by the combination of always stemless shoots, leaves arranged distichously, blade always entire, the presence of two stamens, and pollen as monads, among other minor differences. In light of the new information on the morphological variability of sheath, stipules, and leaf blade, the validity of L. sanagaensis may be questioned. The description provided by Cusset (1984), as well as the only collection of this species we could study (Kato et al. CMR134 from the type locality at Nachtigal on the Sanaga river, Cameroon), are both included within the morphological range of variation observed on the Gabonese material. Nevertheless, we consider it premature to stop considering L. sanagaensis as a valid and different species until more material from Cameroon is made available and allows for better comparison. In addition, we believe a phylogenetic analysis should be conducted on this group of species, to help untangle this complex and to confirm the hypothesis of L. aloides as a widespread, variable species.

CAMEROON • Rapids of the Kienké river, ca 50 km east of Grand Batanga [“in den Schnellen des Kienke oder Kribi-Baches, c. 50 km östlich Groß-Batanga”]; Jun. 1911; [2°48’00”N, 10°21’00”E]; 90 m; fl., fr.; Mildbraed 5951; holotype: B, isotype: BM [BM000910386].

Cameroon, Gabon, Republic of the Congo. Kuetegue et al. (2019) as well as Ghogue (2010) mention that this species is present in Equatorial Guinea, but they did not cite collections. We have not been able to find any collection clearly from Equatorial Guinea, aside from Letouzey 15334 from the Ntem river at the border with Cameroon. The presence of this species in this country is nonetheless most probable since it was collected on the Ntem river, and in Gabon. In Gabon, this species is known from one collection (Bogner 761) made at the Kinguélé falls in the Monts de Cristal, but recent inventories at this site revealed it has since disappeared due to the construction of two hydroelectric dams on the Mbé river. We have not been able to retrieve this collection and confirm the identification made by Cusset, although she mentioned it as being deposited in Paris. This species has not yet been found again elsewhere in the Monts de Cristal area, despite recent efforts since 2017. In Gabon, L. bifurcata is also present on the Ogooué river around Lastoursville, as attested by a collection made by Le Testu (7424) in 1929, and nine recent collections made in 2019.

In Gabon, rapids in rivers from ca 50 to 450 m wide, 150–350 m in elevation (up to 600 m in Cameroon). It appears to be quite rare where encountered. Flowers and fruits were collected in January, February, May, June, July, November, and December. Recent Gabonese fertile collections were only made in July. In Gabon, it has been found once near Saxicolella nana, in the Ogooué. At the Kinguélé falls, past collections made by Nicolas Hallé comprised L. letestui, I. cristata, and I. tenax, along with L. bifurcata, but no information on habitat partitioning was collected. In Gabon, this species was mostly found in fast-flowing water.

The recently collected Gabonese material confirmed observations already made by Cusset (1984) concerning the morphological variability of this species, especially the presence of stemless shoots as well as elongated stems. Boupoya 1953 to 1957 and Nguimbit 16 all show only stemless shoots, while Nguimbit 15 shows elongated stems up to 2 cm long. The collection Boupoya 1958 displays elongated stems up to 5 cm long, as well as a few stemless, fertile shoots, on the same individuals. The leaves appear to be variable in shape, most dwarf collections showing no or rarely forked leaves. On individuals with clearly developed stems, most leaves are usually 1–2-times forked. Stipules are also variable, from absent to present, more or less rounded at apex, depending on the material studied, and in variable proportions. Ledermanniella bifurcata appears to generally have two stamens, but in the collection Nguimbit 16, ca 5–10% of its flowers have only one stamen. When it has a dwarf habit, this species is morphologically very close to L. pusilla, except this latter species has pollen as dyads, whereas L. bifurcata has pollen as monads. It is also very similar to another dwarf species with pollen as monads: L. aloides. It can still be differentiated from this species by the combination of at least some leaves being dichotomously divided (vs always entire in L. aloides), and most importantly the conspicuously stalked spathellae, that show a tapered portion at base (stalk) variable in size, up to 7.5 mm long on Boupoya 1957 (vs spathellae never stalked in L. aloides). The numerous collections of L. aloides now allow for a better understanding of its morphological variability and its boundaries, therefore permitting a better selection of truly discriminative characters. It is nevertheless not yet the case for L. bifurcata that, despite its wide range of distribution in Central Africa, remains a rarely collected and poorly known species. Phylogenetic studies may also help in clarifying its position relative to allied species such as L. aloides.

GABON • Waterfall on the Bolo river, a tributary of the Gnyé, itself a tributary of the Ntem [“Cascade de la Bolo, affluent de la Gnyé, qui du Ntem”]; 27 Aug. 1933; [2°07’42”N, 11°45’21”E]; 580 m; fl., fr.; Le Testu 9257; holotype: P [P00179263]; isotypes: BM [BM000910388], BR [BR0000006264734, BR0000009211674], P [P00179264, P00179265], WAG [WAG0194844].

Endemic to Gabon. This species is only known from the type collection, Le Testu 9257, gathered in 1933 at “Cascade de la Bolo, affluent de la Gnyé, qui du Ntem” (waterfall on the Bolo river, a tributary of the Gnyé river, itself a tributary of the Ntem river). The name “Gnyé” was unfortunately misspelled as “Ngounyé” by Cusset (1984), leading to confusion about the locality where this species was collected. The Ngounyé (or Ngounié) river is one of Gabon’s largest, and flows in the southern part of the country, whereas the Gnyé river is a small tributary of the Ntem river, located at the border of Gabon and Cameroon, in the Woleu-Ntem province.

Rapids and falls in rivers ca 30 wide, 580 m in elevation. No information could be found regarding its abundance on the site where it has been found. Flowers and fruits were collected in August. No other Podostemaceae was collected at the type locality.

The northernmost part of Gabon (including the portion of the Ntem river located in the country and its tributaries) has never been explored for Podostemaceae except for this single collection made by Georges Le Testu in 1933. Future explorations in this region could lead to the rediscovery of this enigmatic species, which has surprisingly not been collected in the Ntem river despite recent collecting efforts conducted in Cameroon.

GABON • Douvouca river in Kwango territory [“Terre de Kwango dans la Douvouca”]; 17 Jul. 1908; [2°55’00”S, 11°05’00”E]; 300 m; fl., fr.; Le Testu 1347; holotype: P [P00179329]; isotypes: BM [BM000910389, BM000910390], BR [BR0000006267568], P [P00179330, P00179331].

Equatorial Guinea, Gabon. This species is so far known from four collections only. Two collections were made in southern Gabon by Georges Le Testu in 1908 and 1927 in the Douvouca and Ngounié rivers. In 1968, Nicolas Hallé collected L. letestui at the Kinguélé waterfall on the Mbé river.

Rapids in rivers from ca 10 to 50 m wide, 150–600 m in elevation. No information could be found regarding its abundance on sites where it has been found. Flowers were collected in January and June, fruits in June and July. Inversodicraea cristata, I. tenax, and Ledermanniella bifurcata have been collected at the Kinguélé waterfall, but the precise habitat partitioning occurring at this site before its modification is unknown.

No recent inventories took place in the Douvouca river, and samples gathered from the Ngounié river in 2019 did not allow for the rediscovery of this species, though the type locality has not been visited again. The authors and colleagues visited the Kinguélé waterfall in Monts de Cristal, where Podostemaceae have now disappeared. Despite its relatively large distribution range, L. letestui remains rare.

CAMEROON • Bipindi, in the Lokundje rapids [“Bipinde, an den Lokundje-Schnellen”]; 10 Aug. 1896; [3°05’00”N, 10°25’00”E]; 70 m; fl., fr.; Zenker 1050; holotype: B; isotypes: BM [BM000910383], G [G00014258, G00014259], K [K000959889], L [L0035207, L0035208], M [M0108129], U [U0005651], W [W18980001777], WAG [WAG0002675].

Cameroon, Gabon, Democratic Republic of the Congo. Before 2017, this species was known from nine collections throughout its range, including two from Gabon, in the Ogooué river at Booué and the Ogoulou river at Mitingo. The authors and colleagues have collected this species 119 times since 2017, which makes it the most commonly collected Podostemaceae species of Gabon. In Gabon, L. pusilla is found in all the rivers explored by authors and colleagues in the Monts de Cristal area, as well as the Ogooué, Ngounié, and Louetsi rivers, and the Offooué and Ivindo rivers, near their confluence with the Ogooué. This species is expected to be present in Equatorial Guinea as well as in the Republic of the Congo.

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).

In Gabon, rapids in rivers from ca 10 to 650 m wide, 35–590 m in elevation. It is usually abundant where encountered, forming dense and large mats. Flowers and fruits were collected in July, August, and September, as well as in January, February, and November outside of Gabon. In the Ogooué river, it seems to share micro-habitats with Inversodicraea annithomae, I. thollonii, Ledermanniella aloides, Macropodiella hallaei, and Tristicha trifaria. It is also often the only species found in rapid areas of small forested rivers in the Monts de Cristal area. In Gabon, this species is very ecologically tolerant, being found in slow or fast-flowing water.

The Gabonese material corresponds to the specific concept of Cusset, who already noticed the ability of this species to produce stemless fertile shoots as well as developed stems. The recent collections mostly comprise stemless individuals, but some collections (Boupoya 1751, 1935, and 1936) show elongated stems up to 4 cm long. The root morphology appears also quite variable, with most collections displaying ribbon-like roots up to 3 mm wide, but sometimes also crustose root, up to 8 mm in diameter (Boupoya 1467, for instance). All leaves are linear, varying in length (up to 2.5 cm long in Boupoya 1935), with a good proportion of them being dichotomously divided at least once, the proportion varying depending on the collection. Exceptionally, all leaves can be entire, which brings the question of the validity of L. linearifolia. This species, endemic to southern Cameroon, always shows entire thread-like leaves up to 2 cm and may represent a local form of the more widespread L. pusilla. In addition, both L. linearifolia and L. pusilla belong to the same subclade within the Ledermanniella-Dyad group (see molecular data and illustrations in Moline et al. 2007: fig. 6A–G; Thiv et al. 2009; Koi et al. 2012). While in L. pusilla, most leaves bear stipules, some are also lacking such features, sometimes on single shoots. Exceptionally, some double-sheathed leaves (between two spathellae on a single stemless fertile shoot) can show up to four stipules (Boupoya 1731). Double-sheathed leaves are a peculiarity of many podostemoid members in America and Africa, always positioned at the fork of dichotomously branching stems (see Cook and Rutishauser 2007: 306; Moline et al. 2007: 163). One collection showed a couple of flowers having only one stamen (Boupoya 1725), and a couple of flowers amongst the several hundred observed showed three stigmas instead of two. Despite important morphological variability, L. pusilla can be recognized by the combination of pollen as dyads, leaves distichously arranged, linear, and almost never strictly entire, flowers with (usually) stalked spathellae, almost always bearing two stamens, among other minor characters.

GABON • Samba waterfall in the Ngounié river, near Sindara [“Chutes de Samba, Sindara, dans la Ngounyé”]; 30 Jun. 1917; [1°02’23”S, 10°41’44”E]; 35 m; fl., fr.; Le Testu 2202; holotype: P [P00179351]; isotype: BM [BM000910392].

Endemic to the Ngounié and Louétsi rivers, in Gabon. Before 2017, this species was solely known from the type collection made at Samba (or Tsamba) falls on the Ngounié river, west of Sindara (Le Testu 2202, gathered in 1917). It was found again one century later, in 2018, at three different sites on the Louétsi river, a tributary of the Ngounié, and collected 10 times in 2018 and 2021. Surprisingly, recent inventory at the type locality did not allow to collect this species again, despite the apparent lack of disturbance and immediate threat to this site.

Falls in rivers from ca 35 to 170 m wide, 35–450 m in elevation. It is very localized when encountered, but forming dense, apparently monospecific mats. Flowers and fruits were collected in June and August. In the Louétsi river, it has been found in close vicinity with L. aloides and L. pusilla. This species seems restricted to falls, in fast flowing water, as it was not collected in smaller rapids areas in the Louétsi river.

The presence of both stemless shoots and developed stem up to 8 mm long was already mentioned by Cusset (1984), and is confirmed by the recent observations. This species is recognizable by the combination of ribbon-like root, pollen in dyads, a single stamen, leaves arranged all around the stem (not distichous), that are linear, entire to 3–4 times forked. When a developed stem is present, the leaves at base are usually shorter, sometimes damaged, which can give the impression of stem-scales. Nevertheless, there is a continuum in length and shape between basal and apical leaves, which suggests no true stem-scales (as typical for Inversodicraea) are present in this species.

D.R. CONGO • Stanley Pool; Jun. 1899; [4°12’00”S, 15°33’00”E]; 270 m; fl., fr.; Schlechter 12574; holotype: B; isotypes: BM [BM000910391], BR [BR0000008974969].

Cameroon, Gabon, Republic of the Congo, Democratic Republic of the Congo. Before 2018, this species was known from 29 collections, including 25 from the area of Brazzaville and Kinshasa, in the Congo and Djoué rivers. Three collections are known from Cameroon, in the Edea and Kribi regions, and in between. In 2018, Nicolas Texier collected this species seven times in Gabon, all of them from the Kongou waterfalls on the Ivindo river. It has not been found elsewhere in Gabon. This species is expected to occur in Equatorial Guinea. One collection is reported from Ivory Coast (Aké Assi 5259), but was not identified by Cusset, and likely represents another species.

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).

Falls in rivers from ca 100 to 1800 m wide, 10–460 m in elevation. According to the Gabonese collections, it is very localized when encountered, but forming dense, apparently monospecific mats. At the Kongou falls, it was found in fast-flowing current, but also at the base of falls, on rocks constantly receiving droplets from the falls, located ca 2 m above the water level, and mixed with an undetermined Poaceae species. Flowers and fruits were collected in February in Gabon, and from April to December elsewhere. In the Ivindo river, it has been found in close vicinity with Inversodicraea nicolasii, and Macropodiella hallaei, though apparently not sharing micro-habitats. This species seems restricted to falls, in fast flowing water of the largest rivers (the Congo, Djoué, Ivindo, Kienké, Nyong, and Sanaga rivers), as it was not collected in smaller rapids of those large rivers, nor in smaller rivers. Interestingly, the species was noted as edible on the collection Descoings 5667.

As for other species of Ledermanniella, Inversodicraea, and Macropodiella mentioned in this study, L. schlechteri can also produce flowering individuals without developed stems (Texier 2250, 2251, 2349, and 2445), presumably when the species is not in the ideal conditions (slower-flowing waters). More typical forms, with developed stems, were also collected at the same place (Texier 2272, 2274, 2335). In the absence of stems, L. schlechteri can still be recognized by the combination of having pollen as dyads, two stamens (exceptionally 1 or 3), linear leaves that can be divided, the absence of stipules, and, most importantly, the clearly up to 2 mm spathella stalk. When a stem is present, most of the leaves tend to be forked several times and some may bear long, narrowly triangular stipules, the spathella stalk can be up to 11 mm long, and the pedicel up to 7 mm long after anthesis.

NIGERIA • Ogoja Province: on the Boje-Aboabam path crossing the Afi river; 13 Dec. 1950; [6°12’00”N, 8°56’35”E]; 100 m; fl.; Keay FHI28241; holotype: K, spirit [21952.000]; isotype: BM [BM000910401].

Sierra Leone, Nigeria, Cameroon, Gabon. The type collection (Keay FHI28241) is the only known collection from Nigeria (Afi river, Ogoja Province). Cusset later identified the two Gabon collections Le Testu 7273bis and Le Testu 7413 from the Ogooué river near Lastoursville (see Notes), as belonging to this species. Two collections from Cameroon are cited on GBIF (Thomas 546 and 2655, this latter under a wrong number) but the identifier is unknown. Since 2017, four collections were made in Gabon that most probably belong to this species. However, they were made in the Okano river, and no recent collections of this species are known from the Ogooué, despite the recent inventories in the surroundings of Lastoursville.

Falls in rivers from ca 35 to 500 m wide, 50–220 m in elevation. It appears to be very rare, and its ecology and micro-habitats are not yet well understood. Flowers and fruits were collected in June and August in Gabon, and from October to December elsewhere. In the Okano river, it has been found in close vicinity of Inversodicraea tenax, Saxicolella nana, and Tristicha trifaria, though it is unclear whether they share similar micro-habitats.

The taxonomic concept associated with this species remains unclear. Examination of the two older collections Le Testu 7413 and Le Testu 7273bis revealed that they lack the typical phyllotaxy of L. tenuifolia, as having leaves distichously arranged, whereas Cusset described them as whorled. Moreover, some leaves of the two Le Testu collections were forked, which is inconsistent with Taylor (1953) and Cusset (1974). In that sense they come close to dwarf members of L. bifurcata. However, we have not been able to examine the type collection of L. tenuifolia for confirmation. For now, and until more material is available, we choose to follow Cusset’s conception and determinations of this species, that include the Gabonese collections Le Testu 7413 and Le Testu 7273bis. We believe this name best accommodates for the recent collections made in the Okano river, even though those show longer leaves than described by Cusset (up to 7 mm long in Boupoya 2424), and lack the typical widening at base. Leaves are nonetheless arranged in a whorl, which is coherent with the original description of L. tenuifolia.

GABON • Ogooué river at Booué [“Lit de l’Ogooué à Booué”]; 29 Jul. 1966; [0°05’24”S, 11°55’59”E]; 165 m; fl., fr.; N. Hallé & A. Le Thomas 203; holotype: P [P00179111]; isotypes: BR [BR0000009211575], P [P00179112, P00179113, P00179114], WAG [WAG0194843].

Endemic to the Ogooué and Ivindo rivers, in Gabon. Before 2018, this species was solely known from three collections: Hallé 203 (the type collection) from the Ogooué rapids at Booué, Thollon 728 from the Ogooué rapids at Lopé, and Courtet s.n., supposedly from the Chari or Congo rapids. However, collections attributed to Henri Courtet are highly suspicious with respect to their geographical origin. According to Cusset (1978, see Notes under Macropodiella heteromorpha), Courtet did collect a few specimens during Auguste Chevalier’s 1902–1904 expedition in the “Haut-Chari” region. But it is unsure if Courtet’s collections from the d’Alleizette Herbarium were collected by him or are Chevalier’s collections that were wrongly attributed to Courtet by Charles d’Alleizette. In addition, all collections from Central Africa labelled as Karmann or d’Alleizette are suspected to be nothing more than fragments of other collections made in Central Africa mainly by Georges Le Testu or Théophile Klaine, as it has been proven by examination of the material from other plant families made by Olivier Lachenaud and the authors of this paper. In the case of the Courtet collection of M. hallaei, we can thus not be sure it was indeed collected in the Chari river by Courtet (which would represent a significant and quite surprising extension of the distribution range of the species) or if it is a collection initially collected by Chevalier, or even if it is a fragment of Thollon 728. Until more research is done on the d’Alleizette Herbarium, we prefer to consider the latter as the most probable explanation. Between 2018 and 2021, 22 collections of this species were gathered by the authors and colleagues, mostly at and around the type locality near Booué, in the Ogooué river. In 2018 and 2019, M. hallaei was discovered at two different sites on the Ivindo river: at the Touné falls (located 3.5 km away from the confluence with the Ogooué) and the Kongou falls. Inventories carried out upstream in the Ogooué river, especially around Lastoursville, Poubara, and at the border with the Republic of the Congo, did not yield this species. Macropodiella hallaei is the most commonly found Macropodiella species in Gabon.

Falls and rapids in rivers from ca 390 to 750 m wide, 110–465 m in elevation. This species appears to be very abundant when encountered. It appears to be found mostly in the middle of rivers rather than on the rocky banks, and in fast-flowing water, where it forms dense monospecific mats. At the Booué rapids, it has also been found mixed with I. thollonii, L. pusilla, and Tristicha trifaria. Flowers and fruits were collected in February, July, and August. Old fruits were observed in early September.

The recently collected material is very consistent with Cusset’s original description of M. hallaei (Cusset 1978). Minor morphological variations can nonetheless be reported. Most individuals show elongated stems, but a few stemless fertile shoots were also observed. In addition, a few stigmas are clearly papillose, among most of them being globose (sometimes bilobed) as mentioned by Cusset. Most flowers bear three stamens, but can occasionally have two, or rarely four.

GABON • Rapids in the Ogooué river at Lopé [“Rapides de Lopé”]; Feb. 1887; [0°05’38”S, 11°35’24”E]; 110 m; fl., fr.; Thollon 729; holotype: P [P00179126]; isotypes: P [P00179127, P00179128].

Ivory Coast, Cameroon, Gabon. This species was originally described based on material collected by Thollon in 1885 and 1887 in the Ogooué river, at the Lopé rapids. It is also known from the Ivindo river (Florence 527) and the Ogooué rapids at Booué (N. Hallé 201 and N. Hallé 204). In Cameroon, it is known from the Nyong (at Mbalmayo and south of Makak) and Ntem rivers (Mve’ele and Campo falls). A single collection is known from Ivory Coast: Guillaumet 1569 on the Cavally river. A collection from Courtet, without collection number, is available in Paris, but its locality data are considered as very doubtful (see Notes for M. hallaei). Since 2019, it was collected three times by the authors and colleagues, in the Lenké river (a small tributary of the Ogooué near Booué) and in the Ivindo river (at the Touné falls near its confluence with the Ogooué).

Falls and rapids in rivers from ca 40 to 600 m wide, 40–640 m in elevation. Unlike M. hallaei, this species is widely distributed, but appears to be rare when encountered. It is apparently restricted to micro-habitats submitted to fast-flowing water. In the Lenké river, it has been found mixed with I. thollonii and L. aloides. In Gabon, flowers and fruits were collected in January, February, and July, and in Cameroon in January, February, July, and December. In Gabon, old fruits were observed in early September.

Surprisingly, recent inventories carried out at the Ogooué rapids near Booué in 2019, 2020, and 2021 did not allow to rediscover this species at this site, though having been collected there in 1966 by Nicolas Hallé and Annick Le Thomas, and then noted as being abundantly present. Nevertheless, recent inventories revealed its presence in the Lenké river only 8 km northwest of Booué, and the Touné falls of the Ivindo, 30 km east of Booué. We cannot exclude the possibility of a natural replacement of this species by M. hallaei over time, which shares the same micro-habitats and was recently noted as very abundant at Booué. Such mechanisms are only postulated and would need further studies to be confirmed and understood. Even though M. heteromorpha was not recently collected at Booué, its absence from this large area of rapids (ca 550 × 900 m), difficult to access in their entirety, is not certain. This site does not seem to have suffered from any particular human-induced impact, but it is yet unknown whether past stochastic events (such as exceptional natural disturbances of the seasonal variations of water levels) may have favoured one species over another. It is also possible that in such a dynamic environment, natural species replacement may occur without any human-induced impact or natural stochastic extreme climatic events. Long-term surveys of selected areas should be designed to explore these questions, and the Booué rapids seem a good candidate, being located near the city and comprising eight different species.

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].

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.

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.

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). 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.

MAURITIUS • fl., fr.; Bory de Saint Vincent s.n.; holotype: P [P00632465]; isotype: B [B -W 00085 -01 0].

Tropical Africa, South and Central America, Madagascar, and islands of the western Indian Ocean. Before 2018, this species was known from Gabon from a single collection made by Frans Breteler in the Okano river (in 1978). Since 2018, 46 collections have been made in Gabon, in most of the rivers that were explored by authors and colleagues, to the notable exception of the Ivindo river. Tristicha trifaria is now considered one of the most common Podostemaceae species in Gabon.

Falls and rapids in rivers from ca 20 to 600 m wide, 15–575 m in elevation (in Gabon). This species is widely distributed, and appears abundant and ecologically ubiquitous. It has been found in the largest rivers (Ogooué) as well as in small, forested rivers (Bibaka, Méba, etc.). Apparently, it can form small, monospecific patches, but was often found in association with other Podostemaceae species. In Gabon, flowers and fruits were collected in July, August, and September.

Tristicha trifaria is here considered as a single, very polymorphic and ubiquitous species, as suggested by Cook and Rutishauser (2007). Nevertheless, several subspecies were described, but their validity remains uncertain, and more phylogenetic studies are needed to untangle this complex (Kita and Kato 2004; Koi et al. 2015). A complete synonymy is provided in Cusset and Cusset (1988a) and is not reproduced here. The discovery of the commonness of T. trifaria in Gabon is less surprising than the fact that it was not discovered earlier. This is probably the result of an historic sampling bias towards the larger Podostemaceae species, even during the few inventories targeted on this family. As an example, Nicolas Hallé and Annick Le Thomas collected six species at Booué in 1966, but not T. trifaria, which the authors and colleagues have collected five times at this site in 2019 and 2021.

Endemic to Cameroon. Dicraeanthus africanus Engl. is widely distributed in Cameroon, and D. zehnderi H.E.Hess is restricted to the Sanaga and Mbam watershed.

Rapids and falls in rivers ca 50 to 600 m wide, 5–1050 m in elevation.

Dicraeanthus was mentioned as present in Gabon by Sosef et al. (2006), based on one collection (Reitsma 2431) gathered in 1986 at the Poubara waterfalls on the Ogooué river. The collection was determined by the collector and was not observed by Cusset. This collection was supposedly deposited at WAG but could not be retrieved there. Duplicates of Reitsma collections are also present in NY, but this number could not be retrieved there either. The presence of this genus in Gabon could not be confirmed, despite the fact that the Poubara site was visited again in 2019. This collection by Reitsma might have been misidentified as Dicraeanthus, especially since a species name was not assigned by the collector. Since a hydroelectric dam was built at Poubara between 2008 and 2013, it is also possible that the Dicraeanthus population sampled by Reitsma in 1986 has since disappeared due to disturbance to the local environment, though four other Podostemaceae species were collected there in 2019: Ledermanniella pusilla, L. aloides s.l., Inversodicraea annithomae, and Tristicha trifaria. The presence of the genus Dicraeanthus in Gabon remains uncertain, and we choose to not accept it as present until the collection Reitsma 2431 can be observed by a specialist, or the species is collected in the field.

Sierra Leone, Guinea, Ivory Coast, Cameroon, Democratic Republic of the Congo, Angola. This is, with I. tenax, one of the most widespread species in the genus. However, some collections from the southernmost part of its range may have been misidentified and may represent I. tenax (see Notes). Cheek et al. (2017) mention that all collections from West Africa may represent other taxa, since some material originally identified as such later proved to be separate species, e.g. I. pygmaea and I. harrisii (C.Cusset) Cheek. No complete revision of the available material was made for this study, we thus consider its distribution as mentioned by Cheek et al. (2017) with the exception of its presence in Gabon.

Rapids and falls in rivers, 5–750 m in elevation.

This species was known in Gabon from a single collection, Le Testu 5983, collected in 1926 in the Offooué river (mistakenly mentioned as the Ogooué river by Cusset) on the road from Mimongo to Koulamoutou in the Ogooué-Lolo province, and identified as such by Cusset (1983). However, recent examination of pollen of this material revealed monads, while I. ledermannii is described as having pollen as dyads. This excludes the possibility that this material represents I. ledermannii and we consider this collection to represent I. tenax instead. Inversodicraea ledermannii is therefore excluded from the flora of Gabon for the moment. Its presence in the country remains plausible, as this species seems abundant in Cameroon.

Endemic to the Sanaga and Mbam rivers, in Cameroon.

Rapids and falls in large rivers from ca 300 to 800 m wide, ca 400–450 m in elevation. Flowers were collected in March. This species has apparently never been collected on smaller tributaries of the Mbam and Sanaga rivers, but we are unaware of potential recent inventories.

This species was included in the Flore du Gabon volume dedicated to this family (Ghogue 2018) following preliminary identifications made on material collected by the authors and colleagues in 2017. Nevertheless, additional material collected since revealed the unsuspected morphological variability of L. aloides. This discovery has led us to consider the Gabonese material as belonging to this species rather than to L. sanagaensis. This species is thus excluded from the flora of Gabon.

Liberia, Sierra Leone, Guinea, Ivory Coast. This species is endemic to West Africa, where it seems common and present in multiple watersheds.

Falls in rivers from ca 30 to 300 m wide, 20–520 m in elevation. It appears to be quite common, but not abundant when encountered. It was recorded on medium-sized to large rivers with fast-flowing current. Flowers and fruits were collected in January, February, October, and November. In Liberia, it has been found in the vicinity of Ledermanniella aloides and Tristicha trifaria.

Macropodiella garrettii was not mentioned as being present in Gabon in the available literature (Sosef et al. 2006; Ghogue 2018). Nevertheless, one collection from this country is available at P (Thollon 846), made in the Ogooué river at the Lopé rapids. This collection was identified as M. garrettii by Cusset in 1976, but this information was surprisingly not reproduced in her study of the genus Macropodiella published two years later (Cusset 1978). Recent examination of this material revealed that it does not belong to this taxon, differing from it by having two stigma lobes (vs cristate, occasionally papillose) and a cylindrical capsule (vs laterally flattened). Thollon 846 very likely belongs to L. bifurcata. Macropodiella garrettii is thus excluded from the flora of Gabon.