Three new Critically Endangered Inversodicraea (Podostemaceae) species from Tropical Africa: I. senei, I. tanzaniensis and I. botswana

Three new species of Inversodicraea (Podostemaceae), I. senei from the Memv’ele falls of the Ntem River, Cameroon, I. tanzaniensis from the Ruhudji River near Lupembe, Southern Highlands, Tanzania, and I. botswana from the Kasane rapids on the Chobe River, Botswana, are described and illustrated. All three species are known from single sites. All three are assessed as Critically Endangered using the IUCN standard. With 35 species, Inversodicraea is the most species-diverse genus of Podostemaceae in continental Africa, to which it is endemic. Four species of Inversodicraea are now known to be sympatric and point endemics at Memv’ele Falls in Cameroon where a hydro-electric dam is being constructed, threatening them with global extinction. Equally, I. tanzaniensis is threatened by proposed hydro-electric projects in Tanzania. In contrast, I. botswana is threatened by sewage discharge and also by bridge construction.


Introduction
Inversodicraea Engl. has been resurrected to accommodate those species previously included in Ledermanniella subg. Phyllosma C. Cusset (Thiv et al. 2009;Schenk et al. 2015;Cheek & Haba 2016a). Inversodicraea is distinguished from Ledermanniella by the presence of scale-leaves (absent in Ledermanniella), and by having terete or laterally compressed leaf petioles (in Ledermanniella canaliculate or dorsiventrally compressed) (Cheek et al. 2017a). This paper builds on the recent synoptic account of Inversodicraea which recognised 30 species (Cheek et al. 2017a). Subsequently two more species were discovered, Inversodicraea tassing Cheek, and I. koukoutamba Cheek (Cheek et al. 2019b). Material collected in Cameroon, Tanzania and Botswana represents three additional new species which are described in this paper as I. senei Cheek, I. tanzaniensis Cheek and I. botswana Cheek respectively. This increases to 35 the number of species in Inversodicraea, the most species-diverse genus of Podostemaceae in Africa, ahead of Ledermanniella Engl. which has 30 species.
Approximately 2000 new flowering plant species are described each year (Willis 2017), adding to the estimated 369,000 already known to science (Nic Lughadha et al. 2016) although this total is disputed (Nic Lughadha et al. 2017). Widespread species tend to have already been discovered, so that many newly discovered species are range-restricted and so are much more likely to be threatened, such as those three described in this paper. Evidence-based conservation assessments exist for about 21 -26% of known species, and 30 -44% of these assessments rate the species concerned as threatened (Bachman et al. 2018). This makes it imperative to discover and publish such species so that they can be assessed, and so that, if merited, conservation actions can be taken to reduce their extinction risks, such as through designating and implementing Important Plant Areas (Darbyshire et al. 2017;continuously updated).
Podostemaceae are a pantropical family of annual or perennial herbs placed in Malpighiales in a sister relationship with Hypericaceae (Ruhfel et al. 2011). There are about 300 species globally, in c. 54 genera (Koi et al. 2012). Species numbers are highest in tropical America, followed by Asia, with Africa having c. 90 species. All species of the family are restricted to rocks in rapids and waterfalls of clear-water rivers, and are therefore rheophytes. However this very habitat is being increasingly exploited for hydropower at great risk to the survival of the Podostemaceae they contain (Schenk et al. 2015;Cheek et al. 2015;Cheek & Ameka 2016;Cheek et al. 2017a, b) and these have resulted in documented extinctions e.g. in Guinea Inversodicraea pygmaea G.Taylor (Cheek 2018;Cheek & Magassouba 2018) and I. abbayesii (G. Taylor) Cheek (Cheek & Diop 2018), and in Angola Ledermanniella lunda Cheek (Cheek et al. 2015). Most of the African species of Podostemaceae are narrow endemics, many being species known from only a single waterfall. New discoveries of species are still being made frequently (Schenk et al. 2015;Cheek & Ameka 2008, 2016Cheek & Haba 2016a;Cheek et al. 2015Cheek et al. , 2017aCheek et al. , b, 2019bBeentje 2005;Schenk & Thomas 2004;Rial 2002;Cheek 2003;Kita et al. 2008, and also, recently, a new genus, Cheek & Lebbie 2018).
Molecular phylogenetic sampling of African Podostemaceae has been only c. 30% complete at species level (Cheek et al. 2017a). Thiv et al. (2009) and Schenk et al. (2015, have convincingly shown that Ledermanniella subg. Phyllosma C.Cusset forms a wellsupported clade of species that are sister to the rest of all other African Podostemaceae sampled, comprising Ledermanniella subg. Ledermanniella and, embedded within it, seven smaller genera: Macropodiella, Winklerella, Djinga, Dicraeanthus, Leiothylax, Letestuella andStonesia. Thiv et al. (2009) andSchenk et al. (2015) both advocate resurrecting the generic name Inversodicraea Engl., as the oldest generic name for the species of Ledermanniella subg. Phyllosma and both transfer species formerly included under the last taxon to Inversodicraea. The last seven remaining species names of subg. Phyllosma for which formal transfer was necessary were placed into Inversodicraea in Cheek & Haba (2016a). A synoptic revision of Inversodicraea, with six new species, was recently published (Cheek et al. 2017a).

Materials and Methods
New names were made according to the Code of Botanical Nomenclature (Turland et al. 2018). Names of species and authors follow the International Plant Names Index (IPNI, continuously updated). Herbarium material was examined with a Leica Wild M8 dissecting binocular microscope fitted with an eyepiece graticule measuring in units of 0.025 mm at maximum magnification. The drawings were made with the same equipment with a Leica 308700 camera lucida attachment. The material from Cameroon, Tanzania and Botswana which forms the subject of this paper, was derived from 1) a visit to YA, Cameroon in December 2016 when the second two authors presented the first author with material that they had collected, and; 2) material carried to the first author on loan from PSUB by Frances Murray-Hudson in 2018; 3) an anomalous specimen at K from Tanzania formerly attributed to Ledermanniella tenax (C.H.Wright) C.Cusset. These specimens, their dates, locations, collectors, places of deposit all indicated in the results below, were compared to material of all known species of Inversodicraea. Specimens were inspected from the following herbaria: BM, EA, FHO, HNG, K, P, PSUB, SRGH, WAG, WRSL, YA. WAG and COI specimens were not accessible since they were being digitised or their herbaria were being remodelled during the period of study. Terms used in the description follow Beentje & Cheek (2003). The format of the descriptions follows Cheek et al. (2017a). All specimens seen are indicated "!". The conservation assessment follows the International Union for the Conservation of Nature (IUCN 2012) standard. The main online search address used for retrieving specim e n d a t a f r o m l a b e l s a t P w a s h t t p s : / / science.mnhn.fr/institution/mnhn/collection/p/ item/, p00179355?listIndex=128&listCount=610. However, resolution of images of herbarium specimens was insufficient to confirm specimen identification. Herbarium codes follow Index Herbariorum (Thiers, continuously updated).

Results & Discussion
Here we present the results of our morphological investigation of the materials analysed with the methods presented above, describing each of the three new species concerned, comparing them with their most closely similar species, citing the specimens upon which they are based, and indicating their synonymy, ecology, distribution, etymology and conservation status. Annual rheophytic herb 9 -10 mm tall (in fruit). Root both crustose (c. 4 × 4 mm, shoots mainly from margin - Fig. 1A) and narrowly ribbon-like, c. 0.6 mm wide, 0.1 mm thick, internodes c. 2 mm long, with shoots lateral at nodes opposite (Fig. 1C). Haptera not observed on the specimens cited. Flowering and fruiting shoots 1 -5 mm tall, erect, forking from near the base, branches diverging, 2 -3 (-4) per plant, densely clad in scale-leaves from base to apex. Scale leaves heteromorphic; at base of stem ( Fig. 1G lower scale-leaves) oblong (0.4 -) 0.8 -1.2 × 0.1 -0.2 (-0.3) mm apex with 1 -2 minute lobes at apex; middle portion of stem ( Fig. 1G upper scale-leaf) narrowly oblong 0.9 -1.2 (-1.7) × 0.1 -0.3 (-0.6) mm, the distal third divided into 3 -4 (-6) narrowly oblong or narrowly triangular irregular lobes 0.2 -0.3 (-0.4) × 0.1 -0.2 mm, apices rounded or abruptly acute, sinuses between lobes acute, scale-leaves progressively more slender and lobes longer and narrower towards stem apex; immediately adjacent to spathellum (Fig. 1H) shortly oblong in outline 0.2 -0.4 × 0.1 -0.2 mm, 3lobed, lobes c. 0.01 mm long. Leaves not observed on the specimen, possibly not produced. Spathellae single at axils of stem bifurcations, in bud part concealed in scale-leaves globose c. 1.2 mm diam., dehiscing irregularly, then funneliform. Pedicels 6 mm long. Tepals 2, filiform 0.6 -0.65 mm long, flanking the androecium. Androecium about as long as gynoecium (2.1 -2.4 mm), stamens two, united at base into an andropodium 0.6 mm long. Ovary unilocular, ellipsoid, ± orbicular in transverse section (Fig. 1K), 1.4 × 0.5 -0.55 mm, excluding a ribbed stipe 0.4 mm long, gynophore 0.2 mm long. Stigmas 2, united at base, filiform-cylindrical 0.3 -0.35 mm long, erect. Capsule 6-ribbed, ribs prominent, about as deep as wide, commissural ribs obscure. Seeds ellipsoid c. 0.25 × 0.1 mm. Fig. 1 RECOGNITION. Similar to Inversodicreaea pygmaea G.Taylor in diminutive stature and shape of scale-leaves, differing in having 2 stamens (not 1), the scale-leaves of the mid stem: a) larger, 0.9 -1.2 (-1.7) mm long (not 0.5 -1 mm), b) spreading (not appressed tightly to stem), c) median lobe not strongly demarcated from other lobes (not with median lobe twice as long as the lateral lobes). . "our new species fits the IUCN criteria for Critically Endangered and could soon become extinct because of the dam construction and operation" (Schenk et al. 2015). This applies equally to I. senei at the same site. Schenk et al. (2015) also cite an earlier conservation assessment of another Podostemaceae species at the same site: "In the IUCN (2012) Endangered listing for Inversodicraea annithomae (C.Cusset) Rutis. & Thiv [the Cameroon material referred to is now known to be I. tchoutoi Cheek (Cheek et al. 2017a)] known only from the type locality in Gabon and from the Chutes de Memve'ele, Jean-Paul Ghogue wrote: "The major threat to this species in Cameroon is the dam construction planned in the only site locality of this species, the Ntem Waterfalls at Memve'ele near Nyabizan, 60 km east of Campo" (Ghogue 2017).
Since Inversodicraea senei is only known from a single location, with an estimated "area of occupancy" of 4 km 2 (the preferred grid cell size of IUCN) and with threats as cited above, using IUCN (2012), we here assess its conservation status as CR B1+2ab (iii), that is Critically Endangered. ETYMOLOGY. Named for Valery-Olivier Séné Belinga (1976 -) Cameroonian botanist, formerly of the Herbier National Camerounais (YA), now of US Forest Service, co-ordinator for Cameroon. He is a field botanist, REDD-plus specialist and ecologist. Collector with Eric Ngansop of the only known specimens of Inversodicraea senei. LOCAL NAMES AND USES. None recorded. NOTES. Inversodicraea senei is unusual in the genus for its minute size (9 -10 mm tall when in fruit), in which it is rivalled only by I. pygmaea G.Taylor of Guinea. Both species flower when the stems are only 1 -5 mm tall. They also both have stems completely clad in broadly similar shaped scale-leaves. However, there are numerous points of difference in the scale-leaf size, lobation and heteromorphy (see Table 1), and while I. senei has flowers with two stamens, those of I. pygmaea have only one.
Inversodicraea senei is the fourth species of the genus unique to the Memv'ele Falls, the others being I. achoundongii, I. ntemensis (Y.Kita, Koi, Rutish. & W.Thomas, and I. tchoutoi. Therefore, these falls have the highest species diversity known globally for the genus Inversodicraea, exceeding even that of the Lobé Falls near Kribi (which has three species of Inversodicraea and seven other species of Podostemaceae). Among the three sympatric species, I. senei is most likely to be confused with the similarly small I. achoundongii. The last species however has deltoid (equilaterally triangular), thinly scattered scale-leaves (not rounded, lobed, densely imbricate).
From the field observations and photographs of Xander van der Burgt (pers. comm. to Cheek 2018) at these falls, niche-partitioning takes place among these species as has been reported at other falls in Cameroon occupied by multiple species of Podostemaceae (Cheek et al. 2004). Inversodicrea senei and I. achoundongii, among the smallest species in the genus, occur at higher levels in the river bed, and seem to be short-lived annuals, while I. ntemensis and I. tchoutoi, much larger, subshrubby species occur in the spray zone, lower down the river bank. However more field research is needed to map these species at the site and to clarify this matter.
New species to science are continually being discovered in Cameroon as poorly known areas are   Table 2 for additional diagnostic characters); it differs from I. ledermannii (Engl.) Engl. in the scale-leaves sparse on the proximal 2 = 3 of the principal axis (not densely covered to base); capsule 6-ribbed (not 8-ribbed); stigma lobes 1.2 -1.3 mm long (not 0.4 -0.5 mm); pollen in monads (not in diads) (see Table 3 for additional diagnostic characters HABITAT. On rocks in rapids (presumably); 1700 m altitude.
Inversodicraea tanzaniensis achieves the highest altitude of all members of the genus (1700 m according to the label of the type specimen), exceeding the previously stated upper altitudinal limit of the genus held by I. digitata H.E.Hess in Angola at 1642 m (Cheek et al. 2017a: 129). The vast majority of the species of the genus are lowland, occurring below 800 m alt. (Cheek et al. 2017a). Seventeen days before collecting 1131A, Schlieben collected the type of another rheophytic species, the more widespread (Angola to Tanzania) Hydrostachys insignis Mildbr. & Reimers (Hydrostachyaceae)(https:// p l a n t s . j s t o r . o r g / s t a b l e / v i e w e r / 1 0 . 5 5 5 5 / al.ap.specimen.b%2010%200160108) and it is possible that the two species grew together since the name of the river, Ruhudje (now Ruhidji) is the same and the town cited is again Lupembe. However, given the separation in time, and the fact that the last specimen is given an altitude of 1600 m not 1700 m, this is far from certain. CONSERVATION STATUS. Known from the type specimen only. Although the general area is given on the label, and although Lupembe is a famous collecting locality of Schlieben's with many types of other species recorded, the exact site of the Schlieben rapids or falls on the Ruhidji River (also spelled Ruhidge, Ruhudji and Ruhiji according to Polhill 1988), is unknown. The general area is now intensively cultivated for tea (Camellia sinensis L.) as can be seen on Google Earth. This gives concern that the river that in 1931 hosted Podostemaceae might by now have lost many or possibly all of these plants due to surface run-off. Silt and algal growth due to oligotrophic conditions resulting from addition of nutrients from silt are anathema to Podostemaceae. In addition, a dam for a 358 MW hydroelectric project is planned on the Ruhidji at a site c. 75 km E of Njombe (Anon. 2011;World Bank 2011). Such hydroelectric projects have resulted in extinction of Podostemaceae species at other sites in Africa (see introduction). When the Ruhidji dam is constructed, a total of 38 km length of the river, both above the dam and below, will be altered hydrologically (World Bank 2011) and so rendered unsuitable for Podostemaceae survival. Funding for the dam and other power generating projects is still being sought in Tanzania (Akinocho 2018). Apart from this major hydro project, micro hydro projects are planned in Tanzania, one of which is mapped on the Ruhidji at a white-water water length of the river (viewed on Google Earth Jan. 2020) immediately south of Njombe at 9.397 S, 34.7 E (Dotet 2018) which is also potentially a site for Inversodicraea tanzaniensis.
The site of the type location may be (or may have been) 9°18'30.66"S, 35°11'19.7"E (1506 m elevation reading from Google Earth). Here, 6 km from Lupembe, a set of rapids can be seen on Google Earth. However, Polhill (1988) records as a botanical collection locality for Eggeling and others (although Schlieben is not mentioned) the Ruhidji Falls at 9°21'S, 34°45'E which is the proposed location for the micro hydro project discussed above, and which may also be at risk from flooding from the dam of the proposed major hydro project at Ruhidji. Fieldwork is needed to visit the area to attempt to rediscover this species and assess the threats and develop a conservation management plan, if indeed Inversodicraea tanzaniensis still survives.
In recent decades considerable effort has been invested by numerous competent botanists (e.g. Mbago, Ndangalasi, Vollesen, Bidgood, Goyder, Darbyshire, Gereau, Lovett, Luke) in botanical inventory in the Southern Highlands of Tanzania, yet this species has not been recollected. This suggests that it may either be extinct, or that it is extremely rare and local. ETYMOLOGY. Named for Tanzania, being both unique to that country and the only species of the genus known in Tanzania. LOCAL NAMES AND USES. None recorded. NOTES. Inversodicraea tanzaniensis has the distinction of being the only species of the genus known from E. Africa (both Uganda and Kenya lack Inversodicraea). Although Cusset (1983) listed numerous specimens of I. tenax as occurring in Tanzania, these specimens are in fact all from Zambia at the Victoria Falls (with the exception of Schlieben 1131A which is I. tanzaniensis). Cusset (1983) treated Schlieben 1131A as Inversodicraea tenax (as (Ledermanniella tenax (C.H.Wright) C.Cusset) and was followed by Beentje (2005). However, this collection lacks the main spot character for I. tenax, that is, the multiple projections from the dorsal surface of the scale-leaves. The two entities also differ in the number of ribs in the fruit capsule (6 vs 8), the Schlieben material lacking the commissural ribs seen in I. tenax. In addition, I. tanzaniensis differs in larger tepals, a shorter gynophore and, much shorter stems. See Table 3 for additional diagnostic characters. The distribution of specimens of the type is largely taken from Cusset (1983: 384).
Ovary ellipsoid (1.3 -) 1.8 mm long, 0.75 mm diam., unilocular. Styles 2, botuliform 0.2 -0.25 mm long, apex rounded, papillate. Fruit 1.5 -1.7 mm long, 0.7 -0.8 mm diam., with 8 equal longitudinal ribs, ribs moderately pronounced, broader than deep; sutural ribs flanked by two incomplete supplementary, straight ribs ( Fig. 3G and 3J) which do not extend to the apex of the fruit; dehiscing by two sutures. Seeds polygonal-oblong c. 0.25 mm long, about as half as long as broad. Fig. 3. At the grid reference given on the type specimen, the River Chobe is about 440 m wide as measured on Google Earth (accessed 8 Oct. 2018). Most of the breadth of the river, at this point, for a length of 840 m, is occupied by four longitudinal lines of rock covered by trees. Between the islands and the banks, a SW to NE diagonal line of rapids is visible, best developed from the southern, Botswanan bank from which they extend about 200 m. This line of rapids is judged here to be the most likely site for Inversodicraea botswana. CONSERVATION STATUS. Inversodicraea botswana is known with certainty from a single site on the Chobe River near the town of Kasane in the NW corner of Botswana. Kasane is the northern entry point to the Chobe National Park, well known for its high density of Kalahari elephants. The Kasane area is heavily dependent on revenue from ecotourism. Therefore, development detrimental to tourism appears not to be encouraged. In addition, Botswana appears to be unusually well-managed from an environmental perspective, especially in a tropical African context. In contrast to the foregoing two species, research for this paper has not yet discovered any overwhelming threats to Podostemaceae (such as hydro-electric dams) at this site in Botswana (see below). However, low level, more insidious threats do exist (F. Murray-Hudson pers. comm. to Cheek, Dec. 2019). The biggest immediate threat may be additional nutrients entering the river such as from sewage overflow from the clinic and potentially from hotels or lodges (sewage, and fertilisers and pesticides from gardens), given the proximity of these facilities to the river. Also, there continues to be considerable disruption to the river flow during the construction of the enormous road bridge linking Botswana with Zambia at Kazangula, where the Chobe and Zambezi Rivers converge (Kazangula Bridge, continuously updated).

RECOGNITION. Differs from all other species of
Therefore, Inversodicraea botswana is here assessed as Critically Endangered, CR B2a,b(i-iv), using the IUCN (2012) standard.
Possibly uniquely among the single-site endemic species of Inversodicraea published to date, I. botswana appears not to be threatened by hydro-electric projects at present. Internet searches showed that Botswana's electrical power is supplied by three coalpowered energy stations (IPFS downloaded Oct. 2018), and by imports. The intention is to increase Botswanan energy supplies not by hydroelectric projects, the principal threat to Podostemaceae species (Cheek et al. 2017a), but from gas and solar sources (USAID 2018). The Kasane site, unusually for tropical Africa, appears free of hydroelectric projects, even though there is a head of 3 -4 m at the site (dropping from 928 m to 924 m, readings from Google Earth). ETYMOLOGY. Named as a noun in apposition for the country of Botswana, to which currently it appears to be both unique and to be the only species of the genus present. LOCAL NAMES AND USES. None recorded. REMARKS (Cheek et al. 2017a). Inversodicraea botswana is unique in the genus for three characterstates: 1) The scale-leaves on the principal axes are strongly and consistently dimorphic. Both narrowly triangular entire scale-leaves, and tridentate scale-leaves occur intermixed (Fig. 3A, D & E). In all other species scaleleaves on the principal axes (if they occur) are monomorphic.
2) The andropodium is absent, even though two stamens are present. In all other species of Inversodicraea (in fact all other African podostemoids) with 2 (-3) stems, the filaments are united into a common structure for part of their length: the andropodium. Although, due to the nature of the material, anthers are not present.
3) In addition to the usual 8 longitudinal ribs, a further four incomplete ribs flank the commissureas in Stonesia.
There is no doubting the identification of the free staminal filaments, subtended as they are by the filamentous tepals ( Fig. 3G & H). The species is also remarkable for the flattened principal stem axes (Fig. 3A) and for the filamentous tepals, which are amongst the longest in the genus, although not exceeding those of Inversodicraea thollonii (Baill.) Cheek (which are 1.2 -1.5 mm long). More complete material of this species is still required.
The affinities of the species may not be with Inversodicraea tenax as the specimen had previously been identified, but with the species of the Angolan plateau to the west. Inversodicraea fluitans H.E.Hess, I. digitata H.E.Hess and I. warmingiana Engl. all share with I. botswana long principal axes that flow in the water, bearing scale-leaves densely on the lateral, spathellae-bearing, spur-shoots. However, none of the Angolan species has scale-leaves on the principal axis nor are these dimorphic as seen in I. botswana, nor have they flowers where the stamens have two, completely free filaments. The principal differences between I. botswana and I. tenax are the complete absence in the first of any of the projections from the dorsal face of the scale-leaf that characterise the second. Similarly, I. tenax lacks all three of the characters described above as being unique to I. botswana.
Botswana, in contrast to Cameroon and Tanzania, has comparatively low plant species diversity and few country endemics. Therefore, the discovery of this species is unexpected.

The first collection of Inversodicraea botswana
The earliest known collected specimen of Inversodicraea botswana is that of Dr Elaine Young s.n. (BM). The material shares the unique combination of tridentate and entire scale-leaves on the stems. However, neither flowers nor fruit are present so the other distinctive features of the species remain undetectable. Yet spathellae are present. The plants were collected three months earlier than the type gathering and perhaps for this reason, being in an earlier state, are smaller in stature, only 2 -4 cm tall. The location given is at the junction of the Linyanti and Zambesi Rivers. However, at the confluence itself no suitable habitat (rapids) can be seen. The nearest sites with suitable habitat are 1) the type locality, 9 km upstream of the confluence on the Linyanti-Chobe River, and 2) 16 km downstream of the confluence on the Zambezi near the settlement of Katambora (on the Zambesi side). Here, at 17°50'13.09"S, 25°22'07.86"E, an extensive area of rapids can be seen on Google Earth (observed 19 June 2018). If this was not the actual site of the Young collection, it is worth surveying in case I. botswana also occurs there. Upstream of the type locality the Chobe-Linyanti and tributaries form a meandering maze of channels, resembling an inland delta, in which no rapids and therefore no suitable Podostemaceae habitat, are present.
The Young specimen is identified as Ledermanniella torrei C.Cusset (now Inversodicraea torrei (C.Cusset) Cheek) by Cusset (1997). This identification is erroneous since I. torrei, a point endemic of Mount Namuli in Mozambique, has monomorphic scale-leaves with three extraordinary long and slender acute lobes. It lacks the mixed, dimorphic scale-leaves (and other distinctive features referred to above) of I. botswana.
Janis Shillito is thanked for typing the manuscript. George Gosline, Frances Murray-Hudson, and two anonymous reviewers are thanked for constructive comments on an earlier draft of the paper.
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