Lukea gen. nov. (Monodoreae-Annonaceae) with two new threatened species of shrub from the forests of the Udzungwas, Tanzania and Kaya Ribe, Kenya

Summary. A new genus, Lukea Cheek & Gosline (Annonaceae) , is erected for two new species to science, Lukea quentinii Cheek & Gosline from Kaya Ribe, SE Kenya, and Lukea triciae Cheek & Gosline from the Udzungwa Mts, Tanzania. Lukea is characterised by a ﬂ attened circular bowl-shaped receptacle-calyx with a corolla of three free petals that give the buds and ﬂ owers a unique appearance in African Annonaceae. Both species are extremely rare shrubs of small surviving areas of lowland evergreen forest under threat of habitat degradation and destruction and are provisionally assessed as Critically Endangered and Endangered respectively using the IUCN 2012 standard. Both species are illustrated and mapped. Material of the two species had formerly been considered to be possibly Uvariopsis Engl. & Diels, and the genus Lukea is placed in the Uvariopsis clade of the Monodoreae (consisting of the African genera Uvariodendron (Engl. & Diels) R.E.Fries, Uvariopsis, Mischogyne Exell, Dennettia Baker f., and Monocyclanthus Keay). The clade is characterised by often conspicuous, ﬁ nely reticulate quaternary nervation, incomplete or absent connective shields (in Annonaceae the connective shield is usually complete) and free petals (except in some Uvariopsis ). Morphologically Lukea is distinct for its broad, turbinate, ﬂ eshy pedicel, a potential synapomorphy within Monodoreae. It appears closest morphologically to the West African monotypic Monocyclanthus , since it shares a trait unusual in the Annonaceae: the calyx in both genera forms a shallow bowl (calyx lobes are absent or vestigial), barely enclosing the base of the petals at anthesis, which persists, living and green, in the mature fruit. However, on recent molecular phylogenetic evidence, Lukea is sister to Mischogyne and the two split c. 20 million years BP , while Monocyclanthus is sister to Dennettia. The placement of Lukea within the Uvariopsis clade is discussed.


Introduction
The Annonaceae is a pantropical family largely restricted to tropical rainforest habitats. The characteristic often showy flowers and fruit help make it widely collected and studied (Chatrou et al. 2012a). Annonaceae are well known for species producing edible fruits (paw paw, soursop, custard apple), and for the fragrant ylang-ylang (Cananga odorata (Lam.) Hook.f. & Thomson) used in perfumes. The Annonaceae is a basal angiosperm family in the order Magnoliales (Heywood et al. 2007). It comprises c. 2500 species and c. 111 genera of trees, shrubs and lianas (Chatrou et al. 2012b;Guo et al. 2017). Within the subfamily Annonoideae, the tribe Monodoreae is composed of 11 genera and 92 species restricted to Africa and Madagascar (Guo et al. 2017;Dagallier 2021).
Sterile, it was initially tentatively identified as possibly a Polyalthia Blume, later as probably a Uvariodendron (Engl. & Diels) R.E.Fries (Monodoreae), although this was doubted by Verdcourt. More than 40 years later Luke collected Luke 4703 (EA, K) also sterile, from Kaya Ribe in the coastal forests of Kenya. Recollected in Sept. 1997 (Luke & Luke 4740 (K) with flower buds and fruit, the latter two specimens were identified as "Annonaceae Genus Nov." by Vollesen at (Luke et al. 9526, 11205, both K) (Figs 3 & 4). Independently of Vollesen, both species were provisionally identified by Luke as Uvariopsis Engl. & Diels. Vollesen was not able to research the material further, being required to complete the Flora of Tropical East Africa account for Acanthaceae before his retirement (Darbyshire et al. 2010). The material at K of his new genus was passed to Gosline in 2017 together with other unresolved East African Annonaceae. Two of these, Marshall 1567 and Cribb & Grey-Wilson 10082, were placed in Mischogyne Exell using molecular methods and prompted a revision of that Afrotropical genus (Gosline et al. 2019). The new genus identified by Vollesen was placed by Gosline in Monodoreae, an entirely African tribe, in the region of Uvariopsis while noting the similarity of venation with Mischogyne. These two genera are placed in a clade together with Uvariodendron and Monocyclanthus Keay (referred to here as the Uvariopsis clade) by Guo et al. (2017) and Gosline et al. (2019). The position remained uncertain until a molecular phylogenetic analysis of the whole Monodoreae tribe using a new set of nuclear markers (Couvreur et al. 2019) was undertaken by Dagallier (Dagallier 2021). Dagallier's phylogenetic study was based on 315 nuclear genes sequenced for almost all (88 of the 92 described) species of the tribe. The results indicated that the sequenced specimens Luke 11205 and 9526 (respectively type and paratype specimens of Lukea triciae) indeed clustered in Monodoreae, and more specifically in the informal "Uvariopsis clade". The Lukea specimens were recovered with maximum support as sister to Mischogyne, the two being sister to the rest of the Uvariopsis clade (Dagallier 2021). However, both Lukea and Mischogyne showed important genetic differentiation (long branches separating both genera which are dated as splitting from each other c. 20 million years BP (Dagallier 2021)), confirming the new genus status as first indicated by Vollesen. To date, no specimen of L. quentinii has been sequenced.
Dagallier's study shows Dennettia Baker f. (sunk into Uvariopsis by Schatz (Kenfack et al. 2003)), as sister to Monocyclanthus, supporting our resurrection of it as a distinct genus.
Twenty-seven genera are accepted in the Flora of Tropical East Africa account of Annonaceae, with 80 named species, and a further 11 which remained informally named (Verdcourt 1971). Lukea keys out in Verdcourt's key to genera for flowering material, to couplet seven, since the carpels are free, and the hairs are simple. However, it fits neither arm of the couplet. While it has three valvate petals, it is not a tree, nor are the carpels 1-ovuled (Enantia Oliv., now Annickia Setten & Maas). Applying the key to fruiting material, it keys to couplet 54 because it has glabrous, nontuberculate, several-seeded monocarps lacking longitudinal ribs with short stipes, seeds lacking pitted faces and far smaller than 2.5 cm long, leaves subglabrous. At this couplet it fails to fulfil both of the two statements since the fruiting pedicel (c. 1.5 mm long), neither exceeds 3 mm (leading to couplet 55) nor is the "fruit practically sessile" (the first arm: Hexalobus A.DC.). Further, in Hexalobus the petals are transversely plicate and united at the base, differing from those of Lukea which are unfolded and free.
In the fifty years since publication of the Flora of Tropical East Africa account (Verdcourt 1971), a further 30 additional new species have been described, bringing the cumulative total of formally named species of Annonaceae to 110 (Vollesen 1980;Verdcourt 1986;Verdcourt & Mwasumbi 1988;Johnson et al. 1999;Deroin & Luke 2005;Couvreur et al. 2006;Couvreur et al. 2009;Couvreur & Luke 2010;Hoekstra et al. 2016;Marshall et al. 2016;Johnson et al. 2017;Johnson & Murray 2018;Gosline et al. 2019;Dagallier et al. 2021;Hoekstra et al. 2021). It seems likely that additional species will continue to emerge for many years so long as taxonomists and botanical inventory work proceed and natural habitat survives and is incompletely surveyed.

Materials and Methods
Herbarium citations follow Index Herbariorum (Thiers, continuously updated). Specimens were viewed at EA (using http://eaherbarium.museums.or.ke/eaherbarium/ results accessed 28 April 2021), BR, K, MO (https:// www.tropicos.org/home accessed 28 April 2021). All specimens cited have been seen by one or more of the authors. Binomial authorities follow the International Plant Names Index (IPNI continuously updated). The conservation assessment was made using the categories and criteria of IUCN (2012). Spirit preserved material was available of both species. 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 drawing was made with the same equipment using Leica 308700 camera lucida attachment. The flowers and fruits of herbarium specimens of the new species described were soaked in warm water to allow dissection. The terms and format of the description follow the conventions of Beentje & Cheek (2003) and Gosline et al. (2019). Georeferences for specimens lacking latitude and longitude were obtained using Google Earth (https://www.google.com/ intl/en_uk/earth/versions/ ). The map was made using QGIS (https://www.qgis.org).

Taxonomic Results
Morphological characters separating Lukea from other genera in the Uvariopsis clade of Monodoreae are given in Table 1 below. Lukea, apart from the fleshy turbinate pedicel (a potential synapomorphy within the tribe which may also occur independently in the unrelated Neotropical genus Fusaea (Baill.) Saff. (Chatrou & He 1999)) has a combination of characters seen in no other genus.
Key to genera of the Uvariopsis clade 1. Quaternary nerves of leaf blades moderately or very conspicuous, forming a fine reticulum; petals & sepals 3merous; flowers bisexual………………………………………………………………………...………………….…..2 1. Quaternary nerves of leaf blades absent or inconspicuous; petals (4) and sepals (2)  those of sterile (non-reproductive) stems longer and larger than those of reproductive stems. Leaf-blades drying green, lacking translucid gland dots, subglossy or glossy above and matt below, ovate, elliptic or ellipticoblong, rarely slightly oblanceolate, acumen short, broadly triangular, base rounded; midrib cylindric on the abaxial surface, visible usually only as a slit on the upper surface. Secondary nerves 6 -19 on each side of the midrib, brochidodromous, arising at c. 60 -80°from the midrib, straight, forking at c. 1 cm from the margin and uniting with the secondary nerves above and below to form an angular inframarginal nerve, connected by lower order nerves to two other parallel and less distinct, inframarginal nerves closer to the thickened, revolute margin. Tertiary and quaternary nerves also raised, as prominent as the secondary nerves, and forming a conspicuous reticulum of isodiametric cells on both adaxial and abaxial surfaces, cell (areolae) diam. c. 1 mm. Indumentum absent or sparse and inconspicuous, along the abaxial midrib and margin, hairs simple, appressed, white. Petioles slightly twisted, dark green (live) drying dull or bright orange, terete but the adaxial surface with a median slit, narrower in width at base and apex, transversely ribbed, glabrous. Inflorescences, axillary, scattered sparsely along leafy or leafless nodes of stems 2 -4 seasons old, 1-flowered, often (Lukea quentinii) with a second inflorescence arising sequentially from the base of the first. Peduncle and rachis together very short, hairs scattered, simple, dark brown, appressed. Bracts 3 -4, distichous, green, persistent, shortly ovate-elliptic, apex acute or mucronate, base sheathing or not, increasing in size towards the pedicel, hairs sparsely scattered. Pedicel fleshy-leathery, yellow-green, turbinate, widening steadily from base to apex, the hairs as in the bracts. Flowers, bisexual. Calyx-receptacular tube, thick, leathery, externally warty, broad and short, shallow bowl-shaped, partly enclosing corolla in bud, indumentum on outer surface of scattered, dark brown, simple hairs, inner surface glabrous. Calyx lobes absent or vestigial, 3, alternating with the petals, triangular, minute, inserted at the edge of the calyxreceptacular tube aperture, centripetal (directed towards the floral axis), persistent (if developed), indumentum as receptacle but denser. Petals 3, green, valvate, thick and fleshy, shallowly concave, appearing nearly flat (horizontal) in bud (petal axis nearly perpendicular to that of floral axis), at anthesis elevated from the horizontal by 40 -70°, triangular, apex acute, base truncate-retuse, sides square, outer surface densely covered in appressed sericeous golden hairs, inner surface glabrous. Torus (raised surface of receptacle on which are inserted the stamens and carpels), domed, wider than long. Stamens c. 150 -180, spirally inserted, arranged in 8 -12 ranks. Stamens latrorse, basifixed, erect, linear, connective and thecae nearly the length of the stamen, connective terminal extension semi-globose (fide van Heusden 1992), black, forming an incomplete cap over the anther cells. Pollen in tetrads. Petals and stamens falling after anthesis leaving the calyxreceptacle cup and carpels exposed, protected in the calyx-receptacular tube. Carpels (1 -) 2 -4, brown, not diverging from each other, longer than stamens, subcylindrical, densely brown pubescent, the hairs patent. Stigma sessile, horseshoe shaped-crescentic, adaxial surface glabrous, outer surface with minute white patent hairs. Ovules numerous, biseriate, parietal. Fruit monocarps 1 (-2) (numbers per fruit unknown in L. triciae), obovoid, subcylindric or narrow ellipsoid, in dried fruit becoming constricted or not between the seeds, pericarp yellow when mature, leathery, glossy, glabrous or very sparsely hairy; shortly stipitate or sessile, glabrous or glabrescent; apex rounded, obscurely bifurcate. Fruiting pedicels slightly accrescent. Calyx-receptacle and bracts persistent, green, not accrescent. Seeds 3 -12 per monocarp, orange-brown discoid, or pale-yellow oblong-elliptic and widest near hilum, hilum large, elliptic, with a ragged, crater-like rim, raphe slightly raised, girdling the seed, testa bony, lacking pitting or sculpture; endosperm conspicuously lamelliform ruminate in longitudinal section, embryo flat, minute.
RECOGNITION. Similar to Monocyclanthus Keay in the flexuose stems, conspicuous fine quaternary reticulate venation, axillary, 1-flowered inflorescences (sometimes cauliflorous in Monocyclanthus), bisexual flowers, and calyx-receptacle with only rudimentary (or no) lobes, persistent and disc-like in fruit; differing in leaves lacking translucid gland dots and Citrus scent, torus broader than long (vs twice or more as long as broad), petals 3 (vs 6), the turbinate fleshy pedicel (vs cylindric), carpels 1 -4 per flower (vs 6 -7). DISTRIBUTION (Cheek et al. 2000;Cheek 2000), also in the genus Afrothismia with A. mhoroana Cheek in the Ulugurus and A. baerae Cheek in Kenyan coastal forests (Cheek 2004a, Cheek 2006Cheek & Jannerup 2006). Numerous other taxa are restricted to the Eastern Arc Mts of Tanzania and the Kenyan Coastal Forests, which together are referred to as EACF (see discussion). HABITAT. Evergreen lowland forest remnants, c. 50 -300 m elevation. CONSERVATION STATUS. Both species are globally threatened (see below under the species accounts). ETYMOLOGY. Named for Mr and Mrs Luke, the Kenyan couple who together have collected many thousands of herbarium specimens in surveys they have organised of surviving natural habitat, principally in Kenya and Tanzania, including between them almost all of the material of this new genus. NOTES. The monotypic Sanrafaelia Verdc. from the Usambara foothills of Tanzania shares a number of morphological similarities with Lukea: shrub habit, ribbed petioles, short pedicel with multiple overlapping bracts, connate sepals, and reduced anther connectives. Previously also included in Monodoreae, it is now placed in a separate tribe and can easily be distinguished in flower by having inner and outer petals both present and united in a single whorl (not with only the outer whorl present and free) (Dagallier 2021).
In our pre-print of this paper we had designated Lukea quentinii as type of the genus ), but as a precautionary measure change this here to L. triciae as only the last was included in the molecular study of Dagallier (2021). However, the two species are so closely similar morphologically that initially they were considered to be one species. The characters separating them are quantitative and not qualitative.
Local names and uses of the species of Lukea are unknown and may not exist since both species are so rare. The pollination biology is also unknown, but we conjecture that beetles may be involved as is usual in Annonaceae in Africa since the petals remain largely closed at anthesis, forming a pollination chamber around the sexual parts (Gottsberger et al. 2011;Gottsberger 2012). Whether thermogenesis occurs as in Uvariodendron (Gottsberger et al. 2011) remains to be discovered.

Placement of Lukea in the Uvariopsis clade
The Uvariopsis clade of the Monodoreae (consisting of the African genera Uvariodendron, Uvariopsis, Mischogyne, Dennettia and Monocyclanthus) is characterised (Table 1) by a tendency (present in most but not all genera of the clade) to conspicuous, finely reticulate quaternary nervation in the leaf-blades (usually absent or inconspicuous in Annonaceae) and incomplete or absent connective shields (in Annonaceae the connective shield is usually complete and is considered plesiomorphic). Within the Monodoreae, the Uvariopsis clade is further characterised by having free petals (not united as in the other clade of the tribe, the Isolona clade (Dagallier 2021)), however, a few species of Uvariopsis do have united petals. Placement of Lukea with this group is supported by possession of all three of these characters. The characters are not seen in all genera of the clade, e.g., in Uvariopsis the fine reticulation is not conspicuous, and a connective shield is present in both Monocyclanthus and Dennettia. The last genus has been sunk into Uvariopsis (Kenfack et al. 2003) but is maintained as a separate genus here since it differs in having a connective shield, and in combining two characters (trimerous perianth and bisexual flowers) otherwise seen separately only in two atypical species of Uvariopsis (Uvariopsis congolana (De Wild.) R.E.Fr., and U. bisexualis Verdc., respectively). Dagallier's phylogenetic trees show Dennettia as sister to Monocyclanthus with high levels of support. Despite Lukea being placed in the Uvariopsis clade as sister to Mischogyne (Dagallier 2021), it lacks the numerous diagnostic features of that genus within the clade, such as the extra-axillary inflorescences, the calyx-sheathed corolla in bud, the strongly reflexed corolla with exserted carpels and the extremely long torus (length: breadth 3+:1) (Gosline et al. 2019). Similarities are the reticulate venation, the stamens without connective shield, and the ellipsoid monocarps in the fruit.
Morphologically the new genus appears closest to the West African monotypic Monocyclanthus, sharing a trait unusual in the Annonaceae: the calyx in both genera forms a shallow bowl (calyx lobes are absent or vestigial), barely enclosing even the base of the petals at anthesis. The bowl persists, green and leathery in the mature fruit (Fig. 2). A broadly similar structure is found in Uvariodendron schmidtii Q. Luke, Dagallier & Couvreur (Dagallier et al. 2021) in which the three calyx lobes are distinct, but appear almost completely connate (the individual lobes can still be clearly discerned) forming a shallow saucer at anthesis, however this structure falls post-anthesis ) unlike in Lukea. The ancestor of Lukea may have had a similar calyx, which we speculate may have evolved by developing greater concavity, more complete loss of the calyx lobes, and persistence in the fruit. The occasional presence of vestigial inner petals in Lukea also suggest a link with Mischogyne and Uvariodendron which alone in the clade have regular inner petals. The function of the calyx-like structure, which may derive more from the enlarged receptacle than the calyx (suggested by the lack of evident calyx lobe connation) apart from protecting the flower in early bud, is unknown. It may be that the same structure in Monocyclanthus arose independently, and that the close similarity with Lukea does not indicate a sister relationship. Monocyclanthus and Lukea also share flexuose leafy twigs (seen intermittently in genera of other tribes e.g. Mwasumbia), and an unusually low number of carpels per flower (1 -4 in Lukea and 6 -7 in Monocyclanthus).
Fruits with constrictions between the seeds are a character state for genera and species in Annonaceae, however at least in the case of Lukea quentinii it is clear that this is an artefact of drying (compare Fig. 1Q with Fig. 2).
Lukea triciae is therefore now on current evidence confined to possibly three (but probably only two) locations separated from each other by 12 km of cultivated land at the foot of the Udzungwa Mts. Since there are only two collections with good geographical metadata, one from each site, we estimate the maximum area of occupation as being 12 km 2 using the grid cells of 4 km 2 preferred by IUCN (2012) and the extent of occurrence as 868 km 2 .
The Magombera Forest Reserve site where the species was discovered in 2003, was highly threatened when it was degazetted from the Selous Game Reserve (now also known as the Nyerere National Park) and not regazetted immediately as was intended at the time. It was part privately owned by the adjacent sugar estate that cleared much of the previous forest in the adjoining Kilombero valley in the 1970s. (https:// www.rainforesttrust.org/urgent-projects/creating-themagombera-nature-reserve-in-tanzania/ Accessed 8 August 2022). The Tanzania-Zambia railway was also cut through the forest in the 1970s. A recent campaign to have Magombera given special protection has been successful. It was gazetted as a Nature Reserve in 2019 and is now subject to fundraising. Since 2008 (imagery viewed via Google Earth) some patches of forest at the margin have been cleared, presenting a threat to the habitat of Lukea triciae. Threats at the Msolwa site are firewood collection (Luke pers. obs.). Assuming that the Mtibwa location has been lost, EOO, AOO and mature individuals have therefore also been lost. Given this, the threats above and small EOO and AOO, we here assess the species as EN B1ab(i-v), B2ab(i-v). PHENOLOGY. Flowers and fruit in July, flower buds in Oct., sterile in December. ETYMOLOGY. Named for key co-collector of this new species, Patricia (better known as Tricia) Luke. She organised the botanical fieldwork logistics for the Udzungwa mountains survey during which flowering and fruiting herbarium material came to light in 2003 and 2005. These specimens were crucial for providing the samples that enabled the molecular phylogenetic placement of the new genus. She has organised the logistics for numerous other botanical surveys and has had a life-long interest in nature and conservation, especially in her native Kenya. VERNACULAR NAME & USES. None are recorded. NOTES. Lukea triciae is more incompletely known than L. quentinii. Only a single monocarp is known, and the few flowers available are not quite at anthesis. The earliest gathering, (Semsei 1520, K, sterile) has leaves of similar size and shape to those of fertile collections and so there is no evidence that there is a different juvenile or non-flowering stage, as seen in L. quentinii (see above). Lukea triciae is confined to Morogoro District, Tanzania, to which numerous other rare and threatened species are globally endemic, e.g. Brachystephanus schleibenii (Mildbr.) Champl. (Champluvier & Darbyshire 2009), and Keetia semsei Cheek & Bridson (Cheek & Bridson 2019) both known from a single collection, and Asystasia asystasioides Darbysh. & Ensermu (Darbyshire & Ensermu 2007).

The Eastern Arc Mountains & Coastal Forests
The two species are separated geographically by c. 525 km, as measured by Google Earth. Both locations are placed in the Eastern Arc mountains and Coastal Forests (EACF). The EACF are found in Tanzania and southern Kenya. They form an archipelago-like phytogeographical unit well-known for high levels of species endemism in many groups of organisms (Gereau et al. 2016). Among the better-known mountain blocks are the Udzungwas, the Ulugurus, and the Usambaras. The Mtibwa Forest reserve at the foot of the Nguu Mts, home of Lukea triciae, is among the least known, least well surveyed, and least protected forest patches. Supported by moist air currents from the Indian Ocean, the surviving evergreen forests of the Eastern Arc Mountains alone have 223 species of endemic tree (Lovett 1998) and are variously stated to have 800 (Tanzanian Forest Conservation Group, undated) or as many as 1500 species (Skarbek 2008) of endemic plant species. In herbaceous groups such as the Gesneriaceae, over 50% of the species for East Africa (Uganda, Kenya and Tanzania) are endemic to the Eastern Arc Mts (Darbyshire 2006) and in the Acanthaceae, there are as many as 100 species endemic to the Eastern Arc Mts (Darbyshire et al. 2010) e.g. Isoglossa bondwaensis I.Darbysh. endemic to the Uluguru Mts (Darbyshire 2009). In terms of documented plant species diversity per degree square, the Eastern Arc Mts are second only in tropical Africa to SW Cameroon in the Cross-Sanaga Interval of W-C Africa (Barthlott et al. 1996;Cheek et al. 2001), which also has the degree square with highest generic diversity (Dagallier et al. 2020). Lukea is an example of a genus endemic to the EACF, just as e.g. Medusandra Brenan (Peridiscaceae, Soltis et al. 2007;Breteler et al. 2015) is endemic to the Cross-Sanaga Interval. Several forest genera have disjunct distributions, being found only in the Cross-Sanaga Interval and in the EACF and not in between, e.g., Zenkerella Taub. and Kupea Cheek (Cheek et al. 2003;Cheek 2004b). The EACF include the sole representatives of plant groups otherwise restricted on the continent to the forests of Guinea-Congolian Africa, e.g., Afrothismia Schltr. and Ancistrocladus Wall. (Cheek & Jannerup 2006;Cheek et al. 2000). Extensive forest clearance within the last 100 -150 years has removed forest from some mountains entirely, and reduced forest extent greatly in others. Since the 1970s more than 12% of these forests have been cleared (Tanzania Forest Conservation Group, undated). Yet new taxa such as these two Lukea species are still being published steadily. Let us hope that their natural sites where not already destroyed can be preserved and their extinction avoided.

Conclusions
Lukea quentinii and L. triciae underline the urgency for identifying and publishing species while they still survive. Threats to such new discoveries for science are ever present, putting these species at high risk of extinction. About 2000 new species of flowering plant have been published by science each year for the last decade or more (Cheek et al. 2020), adding to the estimated 369,000 already documented (Nic Lughadha et al. 2016), although the number of flowering plant species known to science is uncertain ). Only 7.2% of species have been assessed and included on the Red List using the IUCN (2012) standard (Bachman et al. 2019), but this number rises to 21 -26% when additional evidence-based assessments are considered, and 30 -44% of these assess the species as threatened (Bachman et al. 2018). Newly discovered species, such as those reported in this paper, are likely to be threatened, since widespread species tend to have been already discovered. There are notable exceptions to this rule (e.g. Vepris occidentalis Cheek (Cheek et al. 2019) a species widespread in West Africa from Guinea to Ghana). However, it is generally the more localised, rarer species that remain unknown to science. This makes it all the more urgent to find, document and protect such species before they become extinct. Until species are delimited, described and known to science, it is difficult to assess them for their IUCN conservation status and so the possibility of protecting them is reduced (Cheek et al. 2020). Documented extinctions of plant species are increasing, e.g., in coastal forest of Kenya Cynometra longipedicellata Harms may well now be extinct at its sole locality, the Amani-Sigi Nature Reserve (Gereau et al. 2020) and in Tanzania Kihansia lovettii Cheek (Cheek 2004b) at the Kihansi dam site, has not been seen since it was first collected despite targeted searches. There are also examples of species that appear to have become extinct even before they are known to science, such as Pseudohydrosme bogneri in Gabon (Moxon-Holt & Cheek 2021) and in Cameroon Vepris bali Cheek (Cheek et al. 2018). In all cases anthropogenic pressures have been the cause of these extinctions. In both Kenya and Tanzania, on the whole, natural habitat is relatively well covered by a well-planned network of protected areas, but nevertheless natural habitat at some sites with species of high value for conservation has all but disappeared as for the site for first collection of Lukea triciae at Mtwiba (see above). Further investment in prioritising the highest priority areas for plant conservation as Tropical Important Plant Areas (TIPAs, using the revised IPA criteria set out in Darbyshire et al. (2017)) as is in progress in countries such as Guinea, Cameroon, Mozambique and Ethiopia might be extended elsewhere to reduce further the risk of future global extinctions of range-restricted endemic species such as Lukea quentinii and L. triciae.

Acknowledgements
We thank Dr Kaj Vollesen for making available to us the material published in this paper as Lukea, which he had been first to identify as a new genus. We thank Nina Davies for facilitating this. Janis Shillito typed the manuscript. Thomas Couvreur and Leo-Paul Dagallier of University of Montpellier are thanked for discussion, accelerating completion of this paper and Leo-Paul for sharing the results of his molecular phylogenetic research on the Uvariopsis clade. We thank Lars Chatrou and an anonymous reviewer for comments on an earlier version of this paper. Quentin Luke thanks Anthony Githitho of the National Museums of Kenya, Nairobi, and Base Titanium, of Kenya for supporting his visit to Kaya Ribe in May 2021.

Declarations
Conflict of interest. The authors declare no conflict of interest.
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