A checklist of rheophytes of Cameroon

Abstract Rivers in Cameroon were surveyed to collect and document rheophytic plants. Rheophytes are the dominant aquatic macrophytes in tropical river systems, where they are adapted to extreme environments of rushing water (e.g., river rapids, waterfalls and flash floods). Rheophytic plants are useful indicators of river health. However, their habitats are threatened by human activities such as agriculture, plantation development, alluvial mining and dam construction, particularly in tropical countries. In this survey we documented 66 rheophytic species in 29 genera and 16 families. Two ferns, 8 monocotyledons and 56 dicotyledons were listed. Apart from the Podostemaceae family in which all species are rheophytic, the other 15 families have few species which are rheophytic. Five of these families have up to four species and the remaining 10 have only one member as a rheophytic species. The conservation status of each species is assessed and discussed. This work urges botanists, conservationists, and policy makers to do more to protect the habitats of rheophytes and put in place strategies and action plans for the conservation of this important biological group.


Introduction
Rheophyte, a term coined by van Steenis in 1932(van Steenis 1978, is used to describe an aquatic plant which is in nature restricted to swift-running rivers and streams and grows up to flood level, but not beyond the reach of regularly occurring flash floods (van Steenis 1978(van Steenis , 1981. Rheophytes occur worldwide but are found particularly in evergreen rain forests, where they are the dominant aquatic macrophytes in tropical river systems (van Steenis 1978, Quiroz et al. 1997, Ameka 2000, Hoyos-Gomez and Bernal 2018. Members of this biological group of plants are not necessarily taxonomically related, but they show a common adaptation to a restricted ecological habitat or environmental factors (van Steenis 1981, Ameka 2000, Ameka et al. 2002, Hoyos-Gomez and Bernal 2018. Rheophytes are adapted to extreme environments of rushing water by having lanceolate leaves, slender and flexible but tough stems, and strong usually fibrous, root systems (Hoyos-Gomez and Bernal 2018). These plants are generally perennial herbs or shrubs, sometimes small to medium trees, while few grow into tall trees. Two categories of rheophytes are recognized: (i) obligate and, (ii) facultative rheophytes (e.g., Ameka et al. (2002)). Obligate rheophytes are confined to waterfalls, streams and river-beds and banks, and below the flood level. Facultative types are found not only in river-beds but also occur in wet places where they are not subjected to fast-flowing water. In this work, rheophytic plants or rheophytes refer to obligate rheophytes.
Twenty-one rheophytic species, excluding the Podostemaceae, were recognized in tropical Africa, in a worldwide census of rheophytes by van Steenis (1981). Earlier in an assessment of rheophytic plants in South Africa, van Steenis (1978) recognized 7 species, again excluding the Podostemaceae. According to Ameka et al. (2002), in a survey of rivers, for rheophytes, in southern Ghana, from 1994 to 2000, 15 species including four Podostemaceae were recorded. Surprisingly, woody rheophytes were not encountered in the survey by Ameka et al. (2002) although van Steenis (1981) had earlier indicated that half of all rheophytes worldwide are woody. In their work on rheophytes of southern Ghana, Ameka et al. (2002) also reviewed rheophytes of Africa but relied only on records from the literature. Their review revealed that ca. 114 species including 73 Podostemaceae species were documented as rheophytes in Africa. Regarding distribution by country, Cameroon was reported to have 53 rheophytes including 33 Podostemaceae; South Africa 7 rheophytes and 3 Podostemaceae; and Nigeria 19 rheophytes including 4 Podostemaceae (Ameka et al. 2002). The known number of rheophytic plants recorded for Africa (including Cameroon, Ghana, Nigeria and South Africa) probably underestimates the actual number and reflects the degree of paucity of information, and lack of systematic collection effort of Podostemaceae and other rheophytic plants in African countries.
A survey to document and study the rheophytes of Cameroon is important for a number of reasons: (i) rheophytes are poorly known in tropical Africa, including Cameroon, compared to South East Asia and South America, according to van Steenis (1981), (ii) they are the dominant aquatic macrophytes in rivers; and are useful biological indicators of river health, and (iii) the diversity of rheophytes is threatened and some species are in danger of disappearing by the increased landuse practices adjoining the rivers, particularly for agriculture, plantation development, and illegal logging; and in the river courses for alluvial mining (e.g., gold and diamond), and also damming of rivers for hydropower in tropical countries. The construction of dams causes destruction of the habitats of rheophytes, particularly the Podostemaceae.
The survey to enumerate and document the rheophytes of Cameroon was conducted from 2010 to 2014; and the rheophytic species encountered are reported here. It is hoped that this work will stimulate further research on rheophytes across the rest of tropical Africa. We draw attention to the urgent need to stop the destruction of habitats of rheophytes and rather map out strategies and action plans for the conservation of this important biological group.

Materials and methods
The study site A survey of rheophytes was carried out in Cameroon, situated between 2°-13°N and 9°-16°E (Fig. 1). Cameroon is generally divided into three main climatic zones: Equatorial climate zone, (2°-6°N), characterized by an annual average precipitation of 2000 mm, and an average temperature of about 25 °C; the Sudanese climate zone, (6°-10°N), characterized by 5-6 months of dry season with an average temperature of about 22 °C, and 1000 mm of precipitation; and the Sudano-Sahalian climate zone, (10°N-13°N), characterized by 7 months of dry season and 400-900 mm of precipitation (Olivry 1986, Munang et al. 2008. From north to south Cameroon, the vegetation ranges from steppe zone, savannah zone, and to forest zone. The central and western parts of Cameroon are dominated by high mountains and plateaus (Segalen 1967, Suchel 1972, Ndoh et al. 2016. The high western range has peaks which vary in elevation e.g., Mt Etinde (1474 m), Mt Mwoanenguaba (2396 m), Mt Kupe (2050 m), Mt Bamboutos (2740 m) with the highest elevation at Mt Cameroon (4095 m) (Letouzey 1985, Sainge 2017. The Adamawa or Central High plateau reaches up to 1500 m (Suchel 1972). Both the western range and the Central High plateau are the result of volcanic and tectonic activities giving rise to faults, volcanic cones and volcanic lakes. These two sectors constitute the main watersheds of Cameroon's drainage systems. The southern section of the country is dominated by a plateau (500 to 900 m) which gently slopes to the east (Congo basin) but falls by steps to the Atlantic coast (Suchel 1972). While the far north is dominated by the lake Chad Basin (Suchel 1972, Olivry 1986), its southern fringe is the River Benue basin, both of which present a monotonous relief. The narrow coastal zone is marked by unstable swamps, especially from the Wouri-Moungo basins to the Ndian-Akpa-Yafe basins (Suchel 1972, Olivry 1986).

Survey
In documenting the rheophytes of Cameroon, several rivers ( Fig. 1) were visited during the dry season (November-February and July-August in the southern part of the country; and October-April in the northern part of the country). In the dry period, water levels recede in rivers and the majority of plants are in their reproductive phase. Sections of the rivers with rocky substrate were intensively sampled. The rheophytic status of some species were in doubt, particularly those on river banks and edge of rivers, such species and their localities were visited also during the wet season (March -June and July-October in the southern part of the country; May-September in the northern part of the country), when the water flow and level were high. This enabled us to determine whether a particular species is able to withstand spate, and is therefore rheophytic. Notes were taken on the habitat conditions, and characteristic rheophytic features of the species encountered, particularly the leaf shape and form, stem characteristics, and rooting system. Voucher specimens of each taxon collected were deposited in the Cameroon National Herbarium in Yaoundé (YA), [YA, acronym, Thiers (2017)]. Voucher specimens were not collected for every rheophyte species (particularly the Podostemaceae) encountered in the field. This is because such species were common and voucher specimens already exist for them. In cases where voucher specimens were not collected, notes were taken to indicate presence of the rheophytic species at the locality.
Distribution maps of rheophytes of Cameroon were done using georeferenced specimen data derived from specimen labels or available literature, and our own field surveys. The conservation status of each species was assessed by calculating the extent of occurrence (EOO) and the area of occupancy (AOO) in Cameroon using GeoCAT (Geospatial Conservation Assessment tool; Bachman et al. 2011) and applying The IUCN Red List Categories and Criteria, version 10.1 (IUCN 2013, IUCN Standards and Petitions Subcommittee 2017). The AOO was calculated based on a user defined grid cell of 2 km. The number of 'locations' (as defined by IUCN 2017) was calculated with regard to each particular threat, such that a single 'location' may encompass more than one adjacent subpopulation. The term subpopulation is used according to IUCN (2017). The Red Data Book of the Flowering Plants of Cameroon: IUCN Global Assessments Cheek 2011), Ghogue (2011), and the online IUCN Red List (http://www.iucnredlist.org) were consulted while determining the conservation status of the species in this study. Our conservation assessments are yet to be submitted to IUCN and as such these assessments should be treated as "preliminary conservation assessments".

Results
The list of rheophytes identified during the study is presented as a checklist organized by families, and each entry consists of the following: (i) Species name, authority and place of publication (ii) Synonym(s) where applicable (iii) Type, followed by Basionym where applicable (iv) Description (v) Specimens examined (vi) Habitat (vii) Distribution (viii) Conservation status in Cameroon

Checklist of rheophytes from Cameroon
The checklist of rheophytes of Cameroon contains 16 families and 66 species. The rheophytic species listed may be placed in two categories: in the first category are families in which few species are rheophytic and in the second category are families in which all species are rheophytic. The former category has 15 families, 17 genera, and 23 species, while the latter category contains only the Podostemaceae family with 12 genera and 43 species.
Families in which few species are rheophytic Pteridophytes (ferns) Lomariopsidaceae Habitat. Rocky riverbeds and streams, and rocky borders of streams and rivers; in evergreen rainforest.
Conservation status in Cameroon. Bolbitis fluviatilis is not listed on http://www. iucnredlist.org nor in Onana and Cheek (2011). The species is currently known from five localities. The extent of occurrence (EOO) is more than 20,000 km 2 , and the area of occupancy (AOO) is about 20 km 2 . Some collecting localities of the species are within the Cameroon Development Corporation (CDC) palm tree plantations, and other localities are proposed for plantation development. Based on this threat, extent and/or quality of the habitat of the species B. fluviatilis is here assessed as Endangered. IUCN Red List Category: Endangered ENB2ab (ii, iii).
Conservation status in Cameroon. Lepidagathis alopecuroides is not listed on http://www.iucnredlist.org nor in Onana and Cheek (2011). The extent of occurrence of this species is estimated at 124,600 km 2 and has an area of occupancy of about 20 km 2 . The taxon is currently known from five localities. Construction of dams are in progress at two sites for this species: a hydroelectric dam at Natchigal on the Sanaga River; and another on the Dja River, and if fully operational the habitat of L. alopecuroides may be destroyed. Based on these threats, and the fact that the species is only known from five localities, L. alopecuroides is here assessed as Endangered. IUCN Red List Category: Endangered ENB2ab (ii, iii). Habitat. Sandy and rocky riverbeds or up to flood level on the bank, in rainforest. Distribution. Cameroon (Fig. 5) and Nigeria. Conservation status in Cameroon. Achyranthes talbotii was assessed by Cheek (2014) globally as Near Threatened (NT) at http://www.iucnredlist.org. The species was, however, assessed by Onana and Cheek (2011) for Cameroon as Vulnerable, since at that time it was known from only 10 sites. The taxon is currently known from about 25 localities. The extent of occurrence is estimated to be above 20,000 km 2 and the area of occupancy is about 100 km 2 . At some localities, road construction and arable farming are in progress but on a limited scale. Although the habitat of the species is under pressure, it does not appear to qualify as threatened under the IUCN red list cri- Habitat. Rocky or sandy riverbeds and by seasonal rivers, in wet evergreen and semi-deciduous rainforests to deserts.
Conservation status in Cameroon. Kanahia laniflora was not assessed by Onana and Cheek (2011). It is listed on http://www.redlist.org as Least Concern (LC) by Lansdown (2013). This is because the species, globally, is believed to be widespread and abundant throughout much of its known distribution area. The species is distributed from Saudi Arabia and Yemen, through Ethiopia and Somalia to Cote d'Ivoire and South Africa. The extent of occurrence of K. laniflora in Cameroon is about 78,700 km 2 ; and area of occupancy of the species is about 32 km 2 . The taxon is currently known from 8 localities in the country. The localities where the species is found are proposed for timber exploitation and plantation development. Based on these threats and the progressive destruction of the vegetation in the localities, extent and/or quality of the habitats, the species is here assessed, as Vulnerable. IUCN Red List Category: Vulnerable VUB2ab (ii, iii). Habitat. Rocky beds of swift-flowing streams and rivers, or wet shrubby and bushy bank of rivers and streams; in rainforest.
Conservation status in Cameroon. Anubias barteri is listed on http://www.iucnredlist.org as Least Concern (LC) in central Africa in 2007 by Ghogue (2010a). The species was not assessed by Onana and Cheek (2011). The taxon is currently known from 16 localities. The extent of occurrence of A. barteri is about 7,600 km 2 and has an area of occupancy of about 64 km 2 . The habitat is mainly threatened by urban development, road constructions and hydroelectric dams already built or at project stage. Despite the threats, and the fact that the habitats are under pressure, the species does not appear to qualify as threatened under the IUCN red list criterion (IUCN 2017). Though human pressure is expected to increase the loss of habitat and reduce area of occupancy and extent of occurrence (EOO), it is not expected that this will be significant. It is possible that the EOO was underestimated because only specimens with geographical coordinates were used to estimate the EOO. Based on these observations, A. barteri is assessed here as Near Threatened. IUCN Red List Category: Near Threatened (NT). Habitat. Forest streams and rivers, submerged during wet season. Distribution. Cameroon (Fig. 9). Conservation status in Cameroon. Cyperus rheophyticus is listed on http://www. iucnredlist.org as Vulnerable in 2017 ). The taxon is currently known from five localities, and endemic to Cameroon. The extent of occurrence of this species is about 3,000 km 2 and area of occupancy is about 20 km 2 . The localities where they occur are proposed for plantation development, timber exploitation and road construction. Not much has changed since the last assessment of Cheek et al. (2017b); here we maintain its status as Vulnerable. IUCN Red List Category: Vulnerable B1ab (iii) +2ab (iii).

Conservation status in Cameroon.
Cyperus cataractarum is not listed on http:// www.iucnredlist.org nor assessed by Onana and Cheek (2011). The taxon is currently known from 6 localities. The extent of occurrence of this species is about 13,400 km 2 and the area of occupancy is about 24 km 2 . The habitat of C. cataractarum is being progressively destroyed by plantation development and timber exploitation. Based on these threats, and extent and/or quality of habitat, the species is currently assessed as Vulnerable. IUCN Red List Category: Vulnerable VUB1+2ab (ii, iii). Habitat. Invariably growing in spray zone of waterfalls, on wet rocks or on river banks, up to flood level, of swift-running water; lowland or submontane evergreen forest, 300-1450 m alt.
Distribution. Cameroon (Fig. 12) and Equatorial Guinea. Conservation status in Cameroon. Plectranthus cataractarum is listed on http:// www.iucnredlist.org. The species was assessed as Vulnerable (Pollard and Paton 2003). Eight years later it was reassessed as Endangered (Onana and Cheek 2011). The taxon is currently known from three collecting localities. The extent of occurrence and the area of occupancy are both estimated to be less than 10 km 2 . The associated threats, such as forest logging and plantation establishment, mentioned by Pollard and Paton (2003) are still ongoing. Based on these threats, and that the habitats are still under pressure, the species is here re-evaluated as Endangered. IUCN Red List Category: Endangered ENB1+2ab (ii+iii). Habitat. Rocks on bed of Minsomo River. Distribution. Cameroon (Fig. 13) and Equatorial Guinea. Conservation status in Cameroon. Calvoa stenophylla is listed on http://www. iucnredlist.org as Endangered (Cheek 2015). The species was assessed as Critically Endangered (Onana and Cheek 2011), since it was known from only one locality and the associated threats at the time. Cheek (2015) reassessed the species, globally as Endangered, since the number of localities has increased to two, and area of occupancy of 8 km 2 . The second locality is in Equatorial Guinea (Cheek 2015). In Cameroon, the species is only known from one locality and the area of occupancy is about 4 km 2 . The extent of occurrence (EOO) is estimated at 1 km 2 , following the IUCN preferred grid cell size for aquatic organisms (IUCN 2013(IUCN , 2017. This was used by Cheek et al. (2015) in assessing the conservation status of Ledermanniella lunda Cheek (Podostemaceae) from Angola. The main threats are forest logging and agriculture activities. Based on these threats, and the continuous decline of vegetation in the area, extent and /or quality of habitat C. stenophylla is here assessed, as Critically Endangered. IUCN Red List Category: Critically Endangered CRB2ab (ii, iii). Habitat. Beds of swift-running rivers; seasonally inundated river banks; rocks at waterfalls, in evergreen rainforest.

Eugenia dusenii
Distribution. Cameroon (Fig. 14), endemic to Cameroon. Conservation status in Cameroon. Eugenia dusenii is not listed on http://www. iucnredlist.org, but it was assessed as Vulnerable VU in Onana and Cheek (2011). The taxon is endemic to Cameroon and currently known from four localities. The area of occupancy is estimated to be about 16 km 2 , and the extent of occurrence is estimated at 4 km 2 . Plantation development and illegal logging of timber are ongoing at the localities. Based on these threats, and the continuous decline of vegetation cover in the area, and extent and /or quality of habitat E. dusenii is currently reassessed as Endangered. IUCN Red List Category: Endangered ENB2ab (ii, iii). Habitat. Riverbeds and earthbanks of shaded forest streams, periodically inundated rocks in rivers; in rainforest.

Oxalidaceae
Distribution. Cameroon (Fig. 15), Liberia and Nigeria. Conservation status in Cameroon. Biophytum talbotii is not listed on http:// www.iucnredlist.org nor assessed by Onana and Cheek (2011). The taxon is currently known from 13 localities. Extent of occurrence of this species is about 38,000 km 2 and the area of occupancy is about 56 km 2 . Forest logging is the main threat at these habitats. Based on this threat and the continuous decline of vegetation cover in the area, extent and /or quality of habitat B. talbotii is here assessed as Vulnerable. IUCN Red List Category: Vulnerable VU B2ab (ii, iii). Habitat. In rock crevices in riverbeds, seasonally flooded; in forest. Distribution. Angola, Cameroon (Fig. 16), Congo, Gabon and Nigeria. Conservation status in Cameroon. Like the species before it, Biophytum zenkeri is not listed on http://www.iucnredlist.org. The taxon is currently known from 9 localities. The extent of occurrence of this species is about 80,000 km 2 and its area of occupancy is about 36 km 2 . Plantation development is in progress at two of the localities and this may affect the survival of the species. Based on this threat, and the continuous decline of vegetation cover in the area, extent and /or quality of habitat B. zenkeri is here assessed as Vulnerable. IUCN Red List Category: Vulnerable VU B2ab (ii, iii). Habitat. Banks of rivers subject to flooding. Distribution. Cameroon (Fig. 17). Conservation status in Cameroon. Pandanus satabiei was not assessed by Onana and Cheek (2011) nor listed on http://www.iucnredlist.org. The taxon is endemic to Cameroon and currently known from five localities. The extent of occurrence of this species is about 2,900 km 2 and the area of occupancy is about 20 km 2 . Human settlements are developing around one collecting locality; also arable farming is in progress along the rivers. Based on these threats, and the continuous decline of vegetation cover in the area, extentand /or quality of habitat, P. satabiei is here assessed as Endangered. IUCN Red List Category: Endangered EN B1+2ab (ii, iii). Habitat. Among rocks, and sandbanks on riverbed or streams. Distribution. Cameroon (Fig. 18), Congo, Côte d'Ivoire, Ghana, Mali, Nigeria, Senegal and Sierra Leone.

Pandanus satabiei
Conservation status in Cameroon. Eragrostis barteri was not assessed by Onana and Cheek (2011), nor listed on http://www.iucnredlist.org. The taxon is currently known from 10 localities some of which are on the Sanaga River. The extent of occurrence of the species is about 260,000 km 2 and area of occupancy is about 44 km 2 . The proposed dam on Sanaga River will destroy some habitats of the species. Based on this threat, and the continuous decline of vegetation cover in the area, extent and /or quality of habitat, E. barteri is currently assessed as Vulnerable. IUCN Red List Category: Vulnerable VU B2ab (ii, iii). Habitat. Inundated sandy or rocky banks of rivers, between rocks in streams or rivers, waterfalls; riverine forest.
Distribution. Cameroon (Fig. 19), Equatorial Guinea and Nigeria. Conservation status in Cameroon. Ixora euosmia was not assessed by Onana and Cheek (2011) nor listed on http://www.iucnredlist.org. The taxon is currently known from 10 localities. The extent of occurrence of this species is about 36,900 km 2 and the area of occupancy is about 44 km 2 . There is a dam built (hydro-electric dam at Songloulou on Sanaga River), and a proposed project, Memve'ele hydro-electric dam at Nyabezan (Ntem waterfall); and mining projects envisaged in Kribi. Based on these threats and the continuous decline of vegetation cover in the area, extent and /or quality of habitat I. euosmia is here assessed as Endangered. IUCN Red List Category: Endangered ENB2ab (ii, iii). Habitat. Rocky bank of rivers, periodically inundated; between rocks in rivers. Distribution. Cameroon (Fig. 20) and Gabon. Conservation status in Cameroon. Ixora inundata is not listed on http://www. iucnredlist.org. However, in Onana and Cheek (2011), the species was assessed as Endangered. The taxon is currently known from five localities. The extent of occurrence of the species is about 56,000 km 2 , and the area of occupancy is about 20 km 2 . The proposed dam on Ntem River is very likely to threaten the survival of I. inundata. Based on this threat and the continuous decline of vegetation cover in the area, extent and /or quality of habitat the species is assessed here as Endangered. IUCN Red List Category: Endangered ENB2ab (ii, iii). (  Habitat. Rocks of riverbeds and inundated banks, up to flood level of streams and rivers, submerged during rainy season.

Virectaria angustifolia
Distribution. Cameroon (Fig. 21), Gabon, Ghana and Nigeria. Conservation status in Cameroon. Virectaria angustifolia is not listed on http:// www.iucnredlist.org, nor assessed by Onana and Cheek (2011). The taxon is currently known from more than 7 localities. The extent of occurrence of the species is about 66,500 km 2 , and its area of occupancy is about 28 km 2 . Timber exploitation and plantation development are in progress at the collecting localities. Based on these threats and the continuous decline of vegetation cover in the area, extent and /or quality of habitat V. angustifolia is here assessed as Vulnerable. IUCN Red List Category: Vulnerable VB1+2ab (ii, iii).  (1430): 302 (1898) Type. Cameroon, Mfoa, rocky bank of Mbei River, 00 Oct 1827, G. L. Bates 527 (holotype: K; isotypes: BM, P).
Habitat. Rocks at banks and beds of rivers and streams. Distribution. Cameroon (Fig. 22) and Gabon. Conservation status in Cameroon. Virectaria salicoides is not listed on http:// www.iucnredlist.org, nor assessed by Onana and Cheek (2011). The taxon is currently known from two localities (Kribi and Ebianemeyong). The extent of occurrence of the species is estimated at less than 100 km 2 , and its area of occupancy is about 4 km 2 . The habitat is threatened by dam construction (Memve'ele Hydro-electric dam on Ntem waterfall at Nyabesan). Based on this threat, and the continuous decline of vegetation cover in the area, extent and /or quality of habitat V. salicoides is here assessed as Endangered. IUCN Red List Category: Endangered EN B2ab (ii, iii). Habitat. Rocky bed of rivers, on banks of streams and rivers. Distribution. Cameroon (Fig. 23).

Conservation status in Cameroon.
Deinbollia angustifolia is listed on http://www. iucnredlist.org as Vulnerable by Cheek (2017a). It was earlier assessed in Onana and Cheek (2011) as Vulnerable. The taxon is endemic to Cameroon and currently known from three localities. The extent of occurrence of this species is estimated less than 100 km 2 , and the area of occupancy is about 12 km 2 . The habitat of D. angustifolia is threatened by illegal timber logging. Based on this threat, and the continuous decline of vegetation cover in the area, extent and /or quality of habitat, the assessment of Cheek (2017a)  Habitat. Inundated rocky bed and bank of rivers or streams. Distribution. Cameroon (Fig 24), Ghana and Nigeria. Conservation status in Cameroon. Deinbollia saligna is listed on www.iucnredlist.org. It was assessed globally as Vulnerable (World Conservation Monitoring Centre, 1998). In Onana and Cheek (2011) the species was assessed for the Red data Checklist of Cameroon as Vulnerable. This taxon is currently known from four localities. The extent of occurrence of the species is estimated at less than 100 km 2 , and the area of occupancy is about 16 km 2 . The localities are threatened by proposals for dam construction and timber extraction. Based on these threats, and the continuous decline of vegetation cover in the area, extent and /or quality of habitat D. saligna is here reassessed as Endangered: IUCN Red List Category: Endangered ENB2ab (ii, iii).

Family in which all species are rheophytic (Podostemaceae)
The present study and herbarium data have shown that 43 species in 12 genera in the Podostemaceae (riverweed) family have so far been documented from the study area. The results revealed six monotypic genera: Leiothylax, Letestuella, Stonesia, Tristicha, Winklerella, and Zehnderia. The genus Inversodicraea has 10 species, Saxicolella has four while three genera Dicraeanthus, Djinga, and Macropodiella have two; and the largest genus in our area Ledermanniella has 17 species. In an earlier work Ameka et al. (2002) recognized 53 rheophytic species including 33 Podostemaceae for Cameroon. Since then 13 species have been added, 10 of which are in the Podostemaceae family. The Podostemaceae added within the last few years are the result of a deliberate search for the riverweed family in Cameroon. If similar efforts are made to collect the rheophytes in other African countries, many more species would be described for the continent. Apart from the Podostemaceae, the Hydrostachyaceae is the only other family in which all species are rheophytic in Africa (Ameka et al. 2002). So far the Hydrostachyaceae has not been recorded in the study area. Species of the two families are important since they are indicators of river health and also are the dominant macrophytes in tropical river systems; contributing to primary production and oxygenation of the river water (Quiroz et al. 1997, Ameka 2000. The Podostemaceae, for example, serve as substrate for epiphytic algae e.g., diatoms and cyanobacteria (blue-green algae) and other microscopic organisms (Ameka 2000). They also are habitats for invertebrate larvae (e.g., larvae of Simulium (black) fly, and nymphs of dragonfly and mayfly) that seek shelter and feed (Quiroz et al. 1997, Ameka per. obs.).

Podostemaceae
Description. Podostemaceae are annuals or perennials, that grow attached to rocks, in fast-flowing water, by rhizoids, or expanded holdfast; resembling algae or mosses. They produce flowers and fruits during the dry season when the water level in the rivers or streams drops. Habitat. River rapids and waterfalls. Distribution. Cameroon (Fig. 25), Gabon. Conservation status in Cameroon. Dicraeanthus africanus has been assessed for the IUCN Red List, globally, in 2007 as Least Concern (Ghogue 2010b). This species was not assessed by Onana and Cheek (2011). Two localities, waterfalls, are known for this species. The extent of occurrence and the area of occupancy are both estimated at less than 10 km 2 . There are currently two major threats to the survival of this species at the Edea and Lobe waterfalls. First is the planned hydropower dam on the Sanaga River. Secondly the waterfalls on Lobe River is a big tourist attraction site. The dam across the river and activities (e.g., trampling) of tourists will adversely impact the habitats of the species and affect its survival. D. africanus is currently reassessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (iii). Habitat. Growing in river rapids and waterfalls of Sanaga. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Dicraeanthus zehnderi was listed on http:// www.iucnredlist.org as Critically Endangered (Ghogue 2010c). It was also assessed by Onana and Cheek (2011) as Critically Endangered since it has not been collected for many decades. The only known collecting locality for D. zehnderi is the waterfall on Sanaga River at Edea. The extent of occurrence and the area of occupancy are both estimated at 4 km 2 . The hydropower dam on the waterfall will adversely affect the survival of this species and other Podostemaceae species present at this locality. The taxon is here reassessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (iii). Habitat. River rapids. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Djinga cheekii is not listed on the http://www. iucnredlist.org. The taxon is known only from the type locality, Mantem River near Manjo. The extent of occurrence is estimated as 4 km 2 , and area of occupancy is about 4 km 2 . The main threat at the locality is agriculture. Based on this threat, and the continuous decline of vegetation cover in the area, extent and /or quality of habitat the taxon is here assessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (iii). Habitat. Mt. Djinga, Adamaoua, near Tignere, river rapids. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. This taxon, D. felicis, has not yet been assessed for the IUCN Red List. This species was assessed by Onana and Cheek (2011) as Critically Endangered since known from a single collection at the time. The species is most likely extinct at the type locality, Djinga Mts, Admmoua, north-western Cameroon. There are two other localities for the species, Juafef waterfall, where a hotel has been built, which is visited by many tourists coming into that area of Cameroon; and the other Anyajua waterfall is in an agricultural landscape, all in NW Cameroon (Ghogue et al. 2009). The extent of occurrence and area of occupancy are both estimated at 4 km 2 each. Tourism and agricultural activities will adversely affect the habitat of the species. D. felicis is, therefore, reassessed currently as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (iii). Habitat. Memve'ele waterfalls, Ntem River, alt. 395 m. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Inversodicraea achoundongii is yet to be assessed for the IUCN Red List. The taxon is currently known only from the type locality at Memve'ele waterfalls on the Ntem River. The extent of occurrence and area of occupancy are both estimated at 4 km 2 each. The proposed hydropower dam on the Ntem River will certainly impact the survival of the species. Based on this threat, the species is here assessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (iii). Habitat. Lobe waterfall, south of Kribi.

Conservation status in Cameroon.
Inversodicraea bosii is listed on http://www. iucnredlist.org. The taxon was assessed as Endangered (Ghogue 2017a). Onana and Cheek (2011) assessed this species earlier as Endangered. The species is known from two localities, Campo waterfalls and Lobe waterfalls at Kribi. The extent of occurrence and area of occupancy are both estimated at 4 km 2 each. Based on the continuing decline in quality of the habitat of the species at Lobe waterfalls due to activities of tourists; and the possible adverse effect of the proposed hydropower dam near Campo waterfalls the assessment of Onana and Cheek (2011) and Ghogue (2017a) cannot be maintained. The species is here reassessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRBI+ B2ab (iii). Habitat. River rapids. Distribution. Angola, Cameroon (Fig. 25), Central African Republic, Equatorial Guinea, Gabon.

Inversodicraea cristata
Conservation status in Cameroon. Inversodicraea cristata is listed on http://www. iucnredlist.org. The taxon was assessed globally as Vulnerable (Ghogue 2017b). The species is known from five localities. The extent of occurrence of I. critata is about 73,144 km 2 and the area of occupancy is about 24 km 2 . The main threats currently known from the localities are mining and agriculture. Based on these threats, and the continuous decline of vegetation cover in the area, extent and /or quality of habitat, I. cristata is currently assessed as Vulnerable IUCN Red List Category: Vulnerable VUB1+2ab (iii). Habitat. On rocks in river rapids. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Inversodicraea ebo is not listed on http://www. iucnredlist.org. The taxon is known only from the type locality. The extent of occurrence and the area of occupancy are both estimated at about 4 km 2 each. The main threats at the locality are forest logging, mining and agriculture. The species is here assessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (iii). Habitat. On rocks in waterfall near the sea, in evergreen forest zone. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Inversodicraea eladii as for the species before it is not as yet assessed for the IUCN Red List. The taxon is known from one locality. The extent of occurrence and the area of occupancy are both estimated at 4 km 2 each. The main threat at the locality is touristic activity. The species is here assessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (iii). Type. Cameroon, Campo, near Dipikar, Aug, Ledermann 440a (YA).

Conservation status in Cameroon.
Inversodicraea kamerunensis is listed on http://www.iucnredlist.org as Vulnerable (Ghogue 2017c). It was assessed by Onana and Cheek (2011) as Critically Endangered. The species is endemic to Cameroon and known from only the type locality; the species has not been collected since 1908. The extent of occurrence and the area of occupancy are both estimated at 4 km 2 each. The main threat at this locality is dam construction on Ntem River. I. kamerunensis is here reassessed and Critically Endangered status maintained. IUCN Red List Category: Critically Endangered CRB1+2ab (ii, iii). Habitat. River rapids. Distribution. Angola, Cameroon (Fig. 25), Côte d'Ivoire, Gabon, Guinea, Sierra Leone.

Inversodicraea ledermannii
Conservation status in Cameroon. Inversodicraea ledermannii is listed on http:// www.iucnredlist.org as Least Concern, globally (Diop 2017). The taxon is known from five localities. The extent of occurrence of I. ledermannii is about 29,454 km 2 and the area of occupancy is about 20 km 2 . The main threats currently known from the localities are logging, agriculture and touristic activities. Based on these threats, and the continuous decline of vegetation cover in the area, extent and/or quality of habitat, I. ledermannii is currently reassessed as Vulnerable. IUCN Red List Category: Vulnerable VUB2ab (iii). Habitat. Rapids of Ntem River. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Inversodicraea ntemensis is not listed on http:// www.iucnredlist.org. It was assessed in Onana and Cheek (2011) as Critically Endangered. The taxon is endemic to Cameroon and to the Ntem River. The extent of occurrence is about 4 km 2 , and the area of occupancy is about 4 km 2 . The main threat to the survival of the species is dam construction on Ntem River. The species is here reassessed and maintained as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (ii, iii). Habitat. Waterfalls in evergreen forest. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Inversodicraea tchoutoi has not yet been assessed for the IUCN Red List. The taxon is known from only the Memve'ele waterfalls. The extent of occurrence is about 2 km 2 , and the area of occupancy is also about 2 km 2 . The main threat is the construction of a dam on the Ntem River and touristic activities. The species is here assessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (ii, iii). Habitat. On rocks in streams. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Inversodicraea xanderi is not listed on http:// www.iucnredlist.org. The taxon is currently known only from Campo. The extent of occurrence is estimated at 4 km 2 , and the area of occupancy is also about 4 km 2 . No major threat is known from the locality where the species occurs, therefore, I. xanderi is currently assessed as Near Threatened. IUCN Red List Category: Near Threatened (NT). Habitat. On rocks in river. Distribution. Cameroon (Fig. 25) and Nigeria. Conservation status in Cameroon. Ledermanniella aloides has been assessed globally as Vulnerable by Diop (2010a). Onana and Cheek (2011) assessed this taxon for Cameroon as Endangered. The taxon is known from one locality. The extent of occurrence, and the area of occupancy are both estimated at 4 km 2 each. Based on the area of occupancy, the number of localities and the agricultural development impact in the area, L. aloides is here reassessed and maintained as Endangered. IUCN Red List Category: Endangered EN B2ab (iii). Habitat. On rocks in waterfalls. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Ledermanniella batangensis is listed on http:// www.iucnredlist.org globally as Critically Endangered (Ghogue 2010d). Onana and Cheek (2011) also assessed this species as Critically Endangered. The taxon is known from only Lobe waterfalls. The area of occupancy and extent of occurrence are both estimated at 4 km 2 . The locality has a booming tourist industry and this has led to a general decline in quality of the habitat of the species. L. batangensis is here reassessed and the earlier assessment is maintained as Critically Endangered. IUCN Red List Category: Critically Endangered B1ab (ii, iii) +2ab (ii, iii). Habitat. River rapids and waterfalls in evergreen forests. Distribution. Cameroon (Fig. 25), Gabon, Congo, Equatorial Guinea. Conservation status in Cameroon. Ledermanniella bifurcata has been assessed globally as Vulnerable (Ghogue 2010e). The taxon is known from 6 localities. The extent of occurrence of L. bifurcata is about 11,166 km 2 and the area of occupancy is about 24 km 2 . The main threats currently known from the localities are forest logging and agriculture. Based on these threats, the number of localities currently known, and the continuous decline of vegetation cover in the area, extent and /or quality of habitat, L. bifurcata is currently reassessed as Vulnerable. IUCN Red List Category: Vulnerable VUB1+2ab (iii). Habitat. River rapids. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Ledermanniella keayi is listed on http://www.iucnredlist.org as Critically Endangered (Diop 2010b). The taxon is known from one locality, restricted to a small area in an agricultural landscape. The extent of occurrence and the area of occupancy are both estimated at 2 km 2 each. The earlier assessment is here maintained as Critically Endangered. IUCN Red List Category: Critically Endangered CRB2ab (iii). Habitat. River rapids and waterfalls in tropical forests. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Ledermanniella letouzeyi is listed on http:// www.iucnredlist.org. The taxon was assessed as Endangered by Cheek (2004). Onana and Cheek (2011) maintained the Endangered status of Cheek (2004). The species is known from two localities. The extent of occurrence is estimated at 4 km 2 and the area of occupancy is about 8 km 2 . The main threats in the locality are forest exploitation and agriculture. The earlier assessment by Cheek (2004) and Onana and Cheek (2011) is maintained. IUCN Red List Category: Endangered ENB2ab (iii). Specimens examined. 7 km south of Kribi, Lobe waterfall, Jan, J. Bos3591 (K); 7 km south of Kribi, Lobe waterfalls, Aug, De Wild 2876 (P, WAG, YA); Nkam, near Sahe, 3 km southwest Nkondjok road Bafang-Yabassi, Feb R. Letouzey 11146 (P).

Ledermanniella linearifolia
Habitat. River rapids and waterfall. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Ledermanniella linearifolia is listed on http://www.iucnredlist.org. It was assessed as Endangered (Ghogue 2010f ). Onana and Cheek (2011) reassessed this species and maintained the Endangered status. The taxon is known from 6 localities. The extent of occurrence of L. linearifolia is about 42,848,649 km 2 and the area of occupancy is about 16 km 2 . The main threats currently known from the localities are agriculture and touristic activities. Based on these threats, the number of localities currently known, and the continuous decline of vegetation cover in the area, extent and /or quality of habitat, L. linearifolia is currently reassessed and maintained as Endangered. IUCN Red List Category: Endangered ENB2ab (iii). Habitat. River rapids. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Ledermanniella monandra has not yet been assessed for the IUCN Red List, but it was assessed in Onana and Cheek (2011). The taxon is known from one locality. The extent of occurrence and the area of occupancy are estimated at 4 km 2 each. Due to habitat degradation and the continuous decline of vegetation cover in the area, extent and /or quality of habitat, L. monandra is currently reassessed as Critically Endangered. IUCN Red List category: Critically Endangered CRB2ab (iii). Habitat. River rapids and waterfalls. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Ledermanniella musciformis is listed on http:// www.iucnredlist.org as Data Deficient (Diop 2010c). Onana and Cheek (2011) reassessed this species as Endangered. This taxon is endemic to Cameroon and known from at least four localities. The extent of occurrence of L. musciformis is about 68,419,636 km 2 and area of occupancy is about 16 km 2 . The main threat currently known from the localities is deforestation and agriculture. Based on these threats, the number of localities, and the continuous decline of vegetation cover in the area, extent and/or quality of habitat, L. musciformis is currently reassessed and Endangered status maintained. IUCN Red List Category: Endangered ENB2ab (iii). Habitat. Perennial waterfalls and river rapids in submontane forest. Distribution. Cameroon (Fig. 25) and Gabon. Conservation status in Cameroon. Ledermanniella onanae is listed on http:// www.iucnredlist.org as globally Endangered (Ghogue 2010g). Onana and Cheek (2011) maintained the Endangered status. The taxon is known from three localities, two of which are on the same river. The extent of occurrence of L. onanae is about 23,751 km 2 and the area of occupancy is about 12 km 2 . The main threats currently known from the localities are forest logging and agriculture. Based on these threats, and the continuous decline of vegetation cover in the area, extent and /or quality of habitat, L. onanae is currently reassessed and the Endangered status maintained. IUCN Red List Category: Endangered ENB2ab (iii). Habitat. Perennial waterfall, in full sun, in deforested area.

Conservation status in Cameroon.
Ledermanniella pollardiana is not as yet assessed for the IUCN Red List. Onana and Cheek (2011) assessed this species as Critically Endangered. The taxon is endemic to Cameroon, and known from only the type locality. The extent of occurrence and the area of occupancy are both estimated at 4 km 2 each. Agricultural activities in the general area of the locality; with increased turbidity and siltation from agricultural practices will adversely affect the species. The species is here reassessed and Critically Endangered status maintained. IUCN Red List Category: Critically Endangered CRB2ab (iii). Habitat. Waterfalls. Distribution. Cameroon (Fig. 25), Democratic Republic of Cong and Gabon. Conservation status in Cameroon. Ledermanniella pusilla is listed on http://www. iucnredlist.org globally as Endangered (Ghogue 2010h). The taxon is known from two localities. The extent of occurrence is about 9,042 km 2 and the area of occupancy is about 16 km 2 . The waterfalls at Lobe are a huge tourist center and the activities have caused a deterioration in the habitat of the species. The species is here reassessed as Endangered. IUCN Red List Category: Endangered ENB1 + 2ab (iii). Habitat. River rapids and waterfalls. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Ledermanniella sanagaensis is listed on http:// www.iucnredlist.org as Critically Endangered (Ghogue 2010i). The taxon is endemic to Cameroon, and known only from the Sanaga waterfall at Natchigal. The extent of occurrence and area of occupancy are both estimated at 4 km 2 each. There is a proposal to build a dam at the locality of the species. Based on this threat the earlier assessment of Ghogue (2010i) as Critically Endangered is maintained. IUCN Red List Category: Critically Endangered B2ab (ii, iii).
Habitat. River rapids and waterfalls. Distribution. Cameroon (Fig. 25), Congo and Democratic Republic of Congo. Conservation status in Cameroon. Ledermanniella schlechteri is listed on http:// www.iucnredlist.org as Vulnerable, globally (Ghogue 2010j). The taxon is known from two localities. The extent of occurrence is less than 100 km 2 , and the area of occupancy is about 8 km 2 . The proposed dam at Edea waterfall will further deteriorate the quality of the habitat of the species at that locality. Base on this threat and the number of localities where the species is currently found, L. schlechteri is here reassessed as Endangered. IUCN Red List Category: Endangered ENB1+2ab (iii). Habitat. River rapids and waterfalls in tropical rain forest. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Ledermanniella thalloidea is listed on http:// www.iucnredlist.org as Endangered (Ghogue 2010k). However, Onana and Cheek (2011) assessed the taxon as Vulnerable. The taxon is endemic to Cameroon and known from two localities. The species' area of occupancy is estimated to be less than 10 km 2 , and the extent of occurrence estimated at 18 km 2 . There is a decline in the quality of the habitat of the species; there is a dam at the Sanaga waterfalls, and another dam construction is in progress at the Nachtigal waterfalls, the two sites for the species. Due to the impact of the dams on the habitat of the species it is here reassessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB2ab (iii). Habitat. River rapids. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Ledermanniella raynaliorum is not listed on http://www.iucnredlist.org. Onana and Cheek (2011) assessed this species as Endangered. The taxon is known from only one locality. The extent of occurrence and the area of occupancy are estimated at about 4 km 2 each. Forest degradation is the main threat at the locality. The species is here assessed as Near Threatened. IUCN Red List Category: Near Threatened (NT). Habitat. River rapids and waterfalls. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Ledermanniella variabilis is listed on http:// www.iucnredlist.org as Endangered (Ghogue 2010l). Onana and Cheek (2011) re-assessed this species as Endangered. The species is known from two localities and the area of occupancy and extent of occurrence are estimated to be less than 10 km 2 each. The Lobe waterfall locality is a famous tourist attraction so there is a continuous decline in the quality of the habitat at this site. The species is here reassessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (ii, iii). Habitat. River rapids and waterfalls in tropical rain forests. Distribution. Angola, Cameroon (Fig. 25), Democratic Republic of Congo. Conservation status in Cameroon. Leiothylax quangensis has been assessed for the IUCN Red List as Endangered (Ghogue 2010m). The taxon is known from one locality, Edea waterfalls. The area of occupancy and extent of occurrence are both estimated at 2 km 2 each. The hydropower dam at Edea will certainly impact the quality of the habitat of the species. L. quangensis is here reassessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (ii, iii). Habitat. River rapids and waterfalls in tropical rain forest. Distribution. Benin, Cameroon (Fig. 25), Central African Republic, Mali, Namibia and Niger.

Letestuella tisserantii
Conservation status in Cameroon. Letestuella tisserantii is listed on http://www. iucnredlist.org as Least Concern (Diop 2010d) since it occurs in many countries. The taxon is known from two localities in Cameroon. The extent of occurrence is less than 100 km 2 , and the area of occupancy is less than 10 km 2 . Dams on the Sanaga River and agricultural activities are the main threats to the species. Based on the threats and the number of localities where the species is found, L. tisserantii is here assessed as Endangered. IUNC Red List Category: Endangered ENB2ab (iii). Habitat. River rapids and waterfalls in tropical rainforests. Distribution. Cameroon (Fig. 25), Côte d'Ivoire and Gabon. Conservation status in Cameroon. Macropodiella heteromorpha is listed on http://www.iucnredlist.org as Vulnerable (Ghogue 2010n). The taxon is known from four localities, and the area of occupancy is less than 500 km 2 and the extent of occurrence estimated at 75 km 2 . At one of the localities (Meve'ele waterfalls) it is proposed to build a hydropower dam. The species is threatened by habitat decline due to future dam construction. According to Ghogue (2010n) two other localities, Nyong River near Mbalmayo and the Mpoume waterfalls on the Nyong River near Makak, have been listed by Cusset (1987) for this species. These two sites have been surveyed by Ghogue but the species has not been seen or collected. M. heteromorpha is, therefore, here reassessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB2ab (ii, iii). Habitat. River rapids and waterfalls in rainforest. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Macropodiella pellucida is listed on http:// www.iucnredlist.org as Endangered (Ghogue 2010o). Onana and Cheek (2011) maintained the Endangered status of Ghogue (2010o). The taxon is endemic to Cameroon and known from two localities. The extent of occurrence is less than 4 km 2 and the area of occupancy of this species is estimated at less than 20 km 2 . There have not been further threats at the habitat of the species since the previous assessment. The species is reassessed as Endangered, maintaining the previous status. IUCN Red List Category: Endangered ENB2ab (iii). Habitat. Submerged on rocks in fast-flowing river. Distribution. Cameroon (Fig. 25) and Nigeria. Conservation status in Cameroon. Saxicolella flabellata is list on http://www. iucnredlist.org. The taxon has been assessed as Data Deficient (Ouedraogo 2010a) since species distribution, population status and threats to the species are unknown at the time of the assessment. Onana and Cheek (2011) reassessed the species as Endangered. The species is found in two localities. The extent of occurrence is estimated at 2 km 2 and the area of occupancy is about 8 km 2 . The main threats at the localities are described as forest exploitation and agriculture. The species is here reassessed as Endangered. IUCN Red List Category: Endangered ENB2ab (iii). Habitat. River rapids. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Saxicolella laciniata is listed on http://www. iucnredlist.org as Vulnerable (Ghogue 2010p). It was assessed in Onana and Cheek (2011) as Endangered. The taxon is known from two localities. The extent of occurrence is estimated at 2 km 2 and the area of occupancy is 8 km 2 . The main threats at the localities are forest exploitation for agriculture purposes. The species is here reassessed and the Endangered status maintained. IUCN Red List Category: Endangered ENB2ab (iii). Habitat. On smooth granite rocks in swift-flowing stream or river. Distribution. Cameroon (Fig. 25) and Nigeria. Conservation status in Cameroon. Saxicolella marginalis is listed on http://www. iucnredlist.org. The taxon has been assessed globally as Critically Endangered (Ouedraogo 2010b). There is only one known collecting locality in the country. According to Ouedraogo (2010b) there is decline in the quality of the habitat due to pollution from laundry operations in the town of Fundong upstream of the site of this species. Based on this threat the assessment of Critically Endangered is maintained. IUCN Red List Category: Critically Endangered B1ab (iii) +2ab (iii). Habitat. River rapids and waterfalls. Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Saxicolella nana is listed on http://www.iucnredlist.org as Vulnerable (Ghogue 2010q). Onana and Cheek (2011) assessed this species as Critically Endangered, since at that time it had not been collected since the first collection many years ago. The taxon is endemic to Cameroon and is known from only Nyong River near Mbalmayo. The species was, however, collected again by Kato and associates in 2007. The area of occupancy of this species can be estimated to be less than 20 km 2 . The extent of occurrence -estimated to be less than 2 km 2 . The main threat is agricultural activity. The species is here reassessed as Endangered. IUCN Red List Category: Endangered ENB2ab (ii, iii). Habitat. Growing in waterfalls, Tello Waterfalls, Ngaoundéré, Adamawa (Cameroon). Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Stonesia ghoguei is listed on http://www.iucnredlist.org as Vulnerable  Habitat. River rapids and waterfalls. Distribution. Cameroon (Fig. 25).

Conservation status in Cameroon.
Winklerella dichotoma is list on http://www. iucnredlist.org as Critically Endangered (Ghogue 2010r). The species is endemic to Cameroon and only known from the Edea waterfalls on the Sanaga River. There is a hydropower dam built on the river at the collecting locality of the species. The area of occupancy and extent of occurrence are estimated at 2 km 2 each. The earlier assessment of Ghogue (2010r) as Critically Endangered is maintained. IUCN Red List Category: Critically Endangered B1ab (iii) +2ab (iii). Distribution. Cameroon (Fig. 25). Conservation status in Cameroon. Zehnderia microgyna is listed on http://www. iucnredlist.org. as Critically Endangered (Ghogue 2010s). The taxon is known from only one locality. The extent of occurrence of Z. microgyna and the area of occupancy are estimated at about 4 km 2 each. There is a dam built on the Sanaga River, the habitat of the species. Based on that threat, and the fact that the species is known only from one locality and the continuous decline of vegetation cover in the area, and extent and/ or quality of habitat, Z. microgyna is here assessed as Critically Endangered. IUCN Red List Category: Critically Endangered CRB1+2ab (ii, iii).

Discussion
The survey of rheophytic plants from Cameroon revealed 66 species distributed in 16 families, and in three major plant groups: 2 ferns, 8 monocotyledons, and 56 dicotyledons (Table 1). Among the monocotyledons only one grass and two sedges were recorded. Within the dicotyledons three shrub/small tree species, Deinbolla saligna Keay (Sapindaceae), Ixora euosmia K.Schaum (Rubiaceae) and Pandanus satabiei Huynh (Pandanaceae) were encountered (Table 1). According to van Steenis (1981), however, about half the species of rheophytes worldwide are trees and thus the paucity of trees in the current survey is surprising. Rheophytic woody plants (e.g., Coffea congensis Froehn and Breonadia salicina (Vahl) Hepper & Wood in the Rubiaceae) are known to occur outside the study area in Africa. Rheophyte diversity, according to some authors e.g., van Steenis (1981) and Hoyos-Gomez and Bernal (2018), is high in South East Asia and South America compared with tropical Africa. A survey of rheophytes of Africa by Ameka et al. (2002) found 53 rheophytes including 33 Podostemaceae species in Cameroon. This means that we have documented 10 more Podostemaceae and three other rheophytic species (from Cyperaceae: Cyperus rheophyticus, C. tonkinensis, and C. cataractarum) for Cameroon within the last 16 years. Further surveys are required across Africa for the full picture of rheophyte diversity and distribution to emerge.
Invariably, the species encountered in the study have characteristic features that adapt them to their peculiar habitats, and enable them to persist in the harsh conditions of swift-flowing water, flush floods, torrents, and waterfalls. The leaves are lanceolate or narrow with a leaf index of at least 3 similar to what van Steenis (1978Steenis ( , 1981 observed while studying the rheophytes of the world. Crinum natans and some other hydrophytes usually have ribbon-like leaves. The rheophytes encountered in Cameroon have firm but flexible stems which can withstand the tearing effect of swift-running rivers and streams. Their roots are strong or mat-rooted to hold them to their various substrates including rocks, gravel and boulders. The habitats of this unique biological group are, however, threatened by human activities. We show that about 36% of rheophytes are Critically Endangered (CR) in Cameroon and only 2% are considered to be of Least Concern (Fig. 26). Fifty three percent of Podostemaceae in Cameroon are in the CR group. There is the need to do more to protect the habitats of the rheophytes, particularly the Podostemaceae in Cameroon.
The habitats of rheophytes in Cameroon and indeed across Africa are threatened by the land use practices around the rivers, and the damming of the rivers for hydroelectric power . One such land use practice, agriculture (arable farming), introduces agro-chemicals and silt to the rivers. This may make the river turbid, thus affecting photosynthetic ability and therefore the productivity of the plants (Ameka 2000). The agro-chemicals may also poison the plants. Timbering loosens the topsoil and during the wet season run-off water carries silt into the rivers; and the effect is the same as for silt from arable farming (Ameka pers. obs.) -turbid water which results in reduced productivity of submerged water plants. Mining is another land use practice affecting the plants and in particular alluvial mining in rivers may contribute silt into the rivers as suggested by Cheek et al. (2015) and Cheek and Lebbie (2018). The amount of silt from alluvial mining could be much higher than from agriculture because that from alluvial mining is generated in-situ, in the riverbed and, therefore, the effect may be more severe. Silt not only reduces photosynthesis efficiency but may also reduce or even prevent establishment of seedlings of rheophytes on rocks, particularly the members of Podostemaceae . In some instances heavy metals such as mercury are used in the recovery of alluvial gold (Afum and Owusu 2016) and these may poison the plants. Indeed Philbrick and Crow (1983) and Philbrick and Novelo (1995) have provided evidence to show that there is a correlation between increased pollution (chemicals) and loss of Podostemaceae populations in South America. The recent upsurge in dam construction effort in many African countries raises concern for the survival of rheophytes. In Cameroon, there are a number of dams built across rivers for hydropower, and efforts are continuing to build many more dams across a number of rivers: (http://www.theworldfolio.com/news/hydroelectric-projects/659/ eroon); examples of these are:, Memve'élé hydroelectric dam on Ntem River(http://www. edennewspaper.net/memveele-hydroelectric-dam-is-60-complete-energy-minister/); Mekin hydro-electric dam on Dja River, Lom-Pangar hydroelectric dam on Lom River (https://www.internationalrivers.org/sites /default/files/attached-files/lp_factsheet.pdf ); Menchum hydroelectric dam on Menchum River, and the Natchigal hydroelectric dam on Sanaga Rive1r at Edea; http://www.hydroworld. com/ articles/2013/11/cameroonmakes-deal-for-330-mw-nachtigal-falls-hydropower-project. html; and https://afrique. edf.com/en/edf-in-africa/news/a-new-phase-for-the-nachtigal-hydroelectric -project).
River rapids, cataracts, and waterfalls, are usually the preferred sites for dam construction for hydro-electric power, and also the habitats for many rheophytes, particularly the Podostemaceae. The rheophytes have become permanently submerged upstream of the dam due to flood water. The plants downstream are subjected to a different threat, that is, the change in flow rate and absence of flash floods below the dam . In Ghana, two collecting localities of Tristicha trifaria (Podostemaceae) on the Volta River are now under lake water. Before the construction of the Akosombo dam on the Volta River in 1965, T. trifaria was collected at Kpando upstream of the dam. A second dam on the Volta River, down-stream of the Akosombo dam, was completed in 1982. T. triticha was collected on rocks in the river rapid just north of the Akuse dam (Ameka 2000). These two collecting sites have been lost because of the localities are permanently submerged (Ameka 2000). Thus dams threaten and /or endanger the very existence of rheophytic plants. We wish to draw attention to the threats posed by alluvial mining and dam construction to the survival of rheophytes and call on conservationists to do more to curb the indiscriminate damming of rivers, and alluvial mining across Africa. They must engage with policy makers in government and suggest alternative livelihoods for alluvial mine workers; and alternative green energy sources e.g., biofuels, biogas, and solar, instead of dam construction for hydropower.