Thesium hispidifructum (Santalaceae), a New Hispidulous Species from Limpopo, South Africa and Notes on Enigmatic T. celatum

: Two shrub-like Thesium species (subgenus Psilothesium ) from the Limpopo and North West Provinces in South Africa are treated. Using morphological information from the literature, herbarium material (including types) and geographic information, specimens of the little-known T. celatum N.E.Br. and a new species, T. hispidifructum N.Lombard and M.M.leRoux sp. nov., are treated. A ﬁrst comprehensive description of the Waterberg–Magaliesberg endemic, T. celatum , is provided along with diagnostic characters to distinguish it from its morphologically most similar-looking species, T. burchellii A.W.Hill. It has hitherto been classiﬁed as data deﬁcient due to taxonomic reasons by the International Union for Nature Conservation (IUCN) but a status of Vulnerable is suggested. Thesium hispidifructum is recognised for the ﬁrst time here and is endemic to Blouberg and Mount Letsjume. It is morphologically most similar to T. disparile N.E.Br. Its name was chosen with reference to its hispidulous to hispid indumentum covering the entire plant including the outside of the ﬂowers and fruit. A threat status of vulnerable is proposed. Treatments of both species include detailed morphological descriptions, information on their distribution and habitat, comparisons and an identiﬁcation key with morphologically similar species and photo plates.

Many Thesium species are known for their problematic and often outdated taxonomy [7,8], which has resulted in Thesium being considered a high priority for taxonomic research in South Africa [9,10]. In addition, several South African Thesium species are known only from type collections-often of poor quality-and are not only enigmatic but also considered data deficient due to taxonomic reasons (DDT) by the International Union for the Conservation of Nature (IUCN).
As part of an ongoing effort to comprehensively revise the genus, taxonomic revisions of four African species groups have been published since 2018 [8,[11][12][13], treating 24 species. To date, the taxonomy of 10 taxa previously listed as DDT on the Red List of South African plants [http://redlist.sanbi.org/, accessed on 15 October 2022] have been resolved. The renewed research interest in Thesium, as well as the continued contribution of citizen scientists have led to the discovery and description of 11 southern African Thesium species new to science since 2018 [8,[12][13][14][15]. This trend indicates that many Thesium species likely remain undiscovered.
The Waterberg region of the Limpopo Province, South Africa contains the Waterberg Biosphere Reserve which was designated a UNSECO site in 2001 (https://en.unesco.org/ biosphere/africa/waterberg, accessed on 23 December 2022). A recent endeavour by several independent parties (e.g., [16]; Waterberg Biodiversity Project lead by Prof. Nigel Barker from the University of Pretoria) to record plants occurring in this area has revealed multiple populations of an unidentified Thesium species. The aim of this study is therefore to determine the identity of this Thesium species, whether it represents a new taxon or not.

Materials and Methods
The identity of the unknown Thesium species was determined through several lines of investigation. First, a thorough study of morphological characters was conducted on fresh specimens received from Manuel Popp and Linda Willemse. Measurements were taken either by hand using a ruler or those less than 3 mm using an Olympus DSX110 digital microscope with Olympus DSX software v. 3.1.9 (Olympus Corporation 2011-2019, Tokyo, Japan). All morphological characters known to be of diagnostic value (habit, growth form, plant indumentum, leaf morphology, inflorescence structure and flower morphology and indumentum), following previous studies (e.g., [6][7][8]12,13,17,18]) and expert knowledge, were studied and noted. Second, an attempt was made to key out the unknown species using two available identification keys and the measurements that were taken in step one [6,7]. Third, a list of Thesium species occurring in similar (bushveld) habitat within South Africa and neighbouring countries was compiled using distribution data from BODATSA (http://posa.sanbi.org/, accessed 2 September 2022), GBIF (https://www.gbif.org/, accessed on 2 September 2022) and iNaturalist (https:// www.inaturalist.org/login, accessed on 2 September 2022). The list included species from South Africa, Namibia, Botswana, Zimbabwe, Zambia and Mozambique. Four, this list was narrowed down by excluding clearly morphologically distinct species that showed a markedly different morphology to the unknown species. Finally, morphological descriptions (from [6][7][8]17]), type specimens (from JSTOR Global Plants; https://plants. jstor.org/, accessed on 12 September 2022) and other herbarium specimens (from K and PRE) of the remaining morphologically similar species were compared to the unknown species. Collection information for specimens of morphologically similar species studied are provided in Supplementary Table S1.
All species circumscriptions presented here are based on a morphological species concept. Author citations follow the International Plant Names Index (https://www.ipni. org/, accessed on 12 September 2022) and herbarium abbreviations follow [19].
Information for the use in Red List status assessments are suggested based on the guidelines provided by the International Union for Conservation of Nature (IUCN Standards and Petitions Subcommittee 2017 [20]). The extent of occurrence and area of occupancy were calculated using GeoCAT software (http://geocat.kew.org/, accessed on 17 November 2022).

Results
A thorough study of all known diagnostically important morphological characters in Thesium, as defined and used in prior studies [3,[6][7][8][11][12][13][14][15]17,18], revealed that six vegetative and 12 reproductive characters (Table 1) are useful when distinguishing among the unknown Thesium species and five morphologically similar species (T. burchellii A.W.Hill, T. gracilarioides A.W.Hill, T. megalocarpum A.W.Hill, T. procerum N.E.Br., and T. whyteanum Rendle). These characters are: (1) maximum plant height, (2) growth form, (3) habit, (4) branching pattern, (5) indumentum, (6) leaf length, (7) inflorescence type, (8) number of flowers per inflorescence, (9) bract length, (10) presence or absence of involucral bracts, (11) pedicel/peduncle length, (12) flower length, (13) hypanthium length, (14) flower lobe length, (15) style length, (16) stamen placement, (17) placental column shape, and (18) fruit stipe length. Using a combination of these characters as well as geographical distribution information, the unknown Thesium species was determined to be the little-known South African species T. celatum N.E.Br. described in 1932 [17]. This study revealed a total of 13 gatherings (13 specimens) of T. celatum (see Additional specimens examined under T. celatum) that were included in this investigation. Table 1. A comparison of the main diagnostic characters to distinguish among Thesium celatum and five morphologically similar species, T. burchellii, T. gracilarioides, T. megalocarpum, T. procerum and T. whyteanum. Missing data are indicated with "?" and uncertain data with "(?)". Although T. celatum is morphologically similar to T. gracilarioides, T. megalocarpum, T. procerum and T. whyteanum, T. celatum is superficially most similar to T. burchellii. Thesium celatum and T. burchellii share tall robust stems (up to 1.2 m tall in T. celatum and up to 1 m tall in T. burchellii), leafy virgate branches, few-flowered inflorescences ((solitary) 2-6-flowered spike-like inflorescences in T. celatum and solitary or subsolitary flowers in T. burchellii) and flowers often subtended by involucral bracts. Thesium celatum generally has longer leaves (2-22(-30) mm long) and bracts (2-15 mm long), larger flowers (3.5-5.6 mm long) with stamens placed in the top half of the hypanthium and longer fruit stipes (up to 2.2 mm long), while T. burchellii has shorter leaves (±3-10 mm) and bracts (±5-7 mm), smaller flowers (2.6-3.6 mm) with stamens placed at the junction between the lobes and hypanthium and shorter fruit stipes (up to ±1 mm long). The placental column structure in T. celatum is undulate while it is unknown in T. burchellii. Based on the limited available information on their geographical distributions, these species are spatially separated. Thesium celatum is known from the Waterberg (Limpopo Province) and one population in the Magaliesberg (North West Province) and T. burchellii from the Northern Cape Province and Botswana. Morphological comparisons with other similar species (T. gracilarioides, T. megalocarpum, T. procerum, T. whyteanum) are provided in Figure 1 and Table 1. hypanthium and shorter fruit stipes (up to ±1 mm long). The placental column structure in T. celatum is undulate while it is unknown in T. burchellii. Based on the limited available information on their geographical distributions, these species are spatially separated. Thesium celatum is known from the Waterberg (Limpopo Province) and one population in the Magaliesberg (North West Province) and T. burchellii from the Northern Cape Province and Botswana. Morphological comparisons with other similar species (T. gracilarioides, T. megalocarpum, T. procerum, T. whyteanum) are provided in Figure 1 and Table 1.  Surprisingly, the quest to identify T. celatum plants brought four additional PRE herbarium collections (see Additional specimens examined under T. hispidifructum) of another unknown Thesium species from Limpopo Province to the authors' attention. Specimens of the second species was mistakenly filed under T. celatum, perhaps due to their similar tall woody habits (a rare character state for the region as mentioned previously). The species in question is however immediately distinguishable from T. celatum and all other Thesium species in the northern provinces by its hispidulous to hispid indumentum which covers the entire plant including the outside of the flowers and the fruit. No species with this unique combination of characteristics is currently known in this region. Using the same methodology described for T. celatum, this second unknown species is believed to be new to science. It is separated from three morphologically similar species (T. disparile N.E.Br., T. rufescens A.W.Hill, and T. transvaalense Schltr.) by a combination of seven vegetative and nine reproductive characters: (1) maximum plant height, (2) growth form, (3) habit, (4) branching pattern, (5) stem diameter, (6) leaf orientation, (7) leaf length, (8) inflorescence type, (9) number of flowers per inflorescence, (10) flower arrangement, (11) flower exterior indumentum, (12) flower length, (13) hypanthium length, (14) flower lobe length, (15) style length, and (16) fruit exterior indumentum ( Table 2). It is seemingly endemic to Blouberg and Mount Letsjume in the Limpopo Province, which is adjacent to the Waterberg region where T. celatum occurs. The name T. hispidifructum N.Lombard and M.M.le Roux is suggested, after its hispid fruit. Table 2. A comparison of the main diagnostic characters to distinguish among Thesium hispidifructum and three morphologically similar species, T. disparile, T. rufescens, and T. transvaalense. Missing data are indicated with "?" and uncertain data with "(?)". Thesium hispidifructum most closely resembles the Cape species T. rufescens in the hairy indumentum and small flowers (2.6-2.7 mm long in T. hispidifructum and ±2.5 mm long in T. rufescens) but differs in the robust (probably erect) stems, straight ascending leaves and raceme-like inflorescences (compared to slender (probably trailing) stems, spreading and recurved leaves and capitate spike-like inflorescences in T. rufescens). These two species are geographically disjunct. Thesium hispidifructum is restricted to the Limpopo Province while T. rufescens is known only from the Western Cape Province. Morphological comparisons between T. hispidifructum and other similar species (T. disparile, T. transvaalense) are provided in Figure 2 and Table 2.

T. hispidifructum T. disparile T. rufescens T. transvaalense
long in T. rufescens) but differs in the robust (probably erect) stems, straight ascending leaves and raceme-like inflorescences (compared to slender (probably trailing) stems, spreading and recurved leaves and capitate spike-like inflorescences in T. rufescens). These two species are geographically disjunct. Thesium hispidifructum is restricted to the Limpopo Province while T. rufescens is known only from the Western Cape Province. Morphological comparisons between T. hispidifructum and other similar species (T. disparile, T. transvaalense) are provided in Figure 2 and Table 2.   Descriptions: Shrub or subshrub, up to 1.2 m tall, rootstock a woody taproot; stems erect to spreading, up to 6 mm in diameter, virgate, glaucous, glabrous below and glabrous to densely verruculose above. Leaves mostly ascending, linear, 2-22(-30) × 0.1-0.75 mm, glabrous to densely verruculose on the margins and midrib or entire surface. Flowers sessile, single in bract axils, occasionally solitary but mostly arranged in 2-6-flowered determinate spike-like inflorescences. Bracts and bracteoles 3-7, leaf-like but smaller, 2-15 mm long, forming an involucre. Perianth narrowly campanulate, 3.5-5.6 mm long, no "glands" observed on outside, externally glabrous; hypanthium 1.0-1.4 mm long; corolla lobes very narrowly triangular (almost linear), 1.2-2.5 mm long, apex slightly cucullate or sometimes somewhat pointed, with a dense apical beard and fringed margins. Stamens inserted in the top half of the hypanthium; anthers 0.4-0.8 mm long, included in-or rarely partially exerted from the hypanthium. Style 0.6-0.9 mm long, stigma often below anthers or sometimes reaching the lower half of the anthers. Placental column slightly to prominently undulate but not twisted; ovules (2)3(4). Fruit globose to elliptic, 6.5-8.4 × 2.4-3.2 mm including stipe and persistent perianth, stipe (0)0.6-2.2 mm long, persistent perianth 2.4-3.1 mm long, longitudinally and reticulately veined when dry, glabrous.
Note: Further useful morphological characters to consider for T. celatum include its stamens that are inserted in the top half of the hypanthium and the presence of undulate placental columns. These characters are unknown in T. burchellii.
Distribution and habitat: Thesium celatum is near endemic to the Limpopo Province in South Africa. It is known from 11 gatherings in the Waterberg area between Grootwater Nature Reserve, Naboomfontein, and Settlers Pride in Limpopo Province, and one isolated gathering in the vicinity of Brits (Nort West Province) (   Phenology and ecology: This species has been found in flower and fruit in January, June, August, September, and November. While Thesium species from this area normally flower between September and February, it is not uncommon for shrub-like species (e.g., T. procerum) to flower throughout the majority of the year ([8], personal observation).

Conservation status:
Thesium celatum has an extent of occurrence (EOO) of 11 560 km 2 and an area of occurrence (AOO) of 36 km 2 . It is known from 13 gatherings at seven to nine locations. Several subpopulations of this species have been observed for the first time in the past two years following field expeditions. It is therefore possible that more subpopulations remain undiscovered. Six of the known gatherings (Jacobsen 2432, Popp s.n., Popp s.n., Popp s.n., Van Vuuren 263 and Willemse 10) came from protected areas. The type gathering (Rogers 24973) came from a farm where most of the natural habitat has been transformed for agricultural purposes and it is very probable that this subpopulation has been lost or are at least severely impacted. Three of the known subpopulations (Burgoyne 1746, Fourie 1512, Van Vuuren 263) are potentially threatened by habitat loss and degradation due to urban and crop developments. The population is therefore believed to be declining. The remaining gatherings come from areas that appear not to be threatened. This species qualifies for a threat status of Vulnerable (VU) under Criterion B1ab(iii,iv,v) + 2ab(iii,iv,v) [20]. Diagnosis: Thesium hispidifructum is morphologically similar to T. disparile in its hairy indumentum, ascending to spreading leaves (4-15 mm long in T. hispidifructum; 7-11 mm long in T. disparile), flowers placed in determinate racemose cymes and flower lobes (0.8-1.2 mm long in T. hispidifructum; ±0.7-1.2 mm long in T. disparile) bearded at the inner apex. Thesium hispidifructum is easily distinguished from T. disparile by its tall robust stature (woody stems up to 1 m tall), intricate branching pattern and compact inflorescences with flowers solitary in the bract axils. Comparatively, T. disparile has a short slender stature (herbaceous stems up to 0.15(0.5?) m tall), virgate branching pattern and lax inflorescences with single flowers usually replaced by compound monochasial or dichasial cymes.

Distribution and habitat:
Thesium hispidifructum is endemic to the Limpopo Province in South Africa, where it is known from only four gatherings between Blouberg and Mount Letsjume in the westernmost reaches of the Soutpansberg (Figure 6). Little is Distribution and habitat: Thesium hispidifructum is endemic to the Limpopo Province in South Africa, where it is known from only four gatherings between Blouberg and Mount Letsjume in the westernmost reaches of the Soutpansberg (Figure 6). Little is known about its habitat. It has been collected among grass in rocky areas as well as in wet wooded areas, on both south facing and northeast facing slopes at elevations between 1675 and 1980 m a.s.l. It is likely restricted to the Soutpansberg Mountain Bushveld [21] and Soutpansberg Summit Sourveld [21], but field studies are needed to confirm this. y 2023, 3,11 known about its habitat. It has been collected among grass in rocky areas as well as in wet wooded areas, on both south facing and northeast facing slopes at elevations between 1675 and 1980 m a.s.l. It is likely restricted to the Soutpansberg Mountain Bushveld [21] and Soutpansberg Summit Sourveld [21], but field studies are needed to confirm this. Phenology: Thesium hispidifructum has been collected in flower and fruit in January, February, June, and July.
Etymology: The specific epithet hispidifructum refers to the hispidulous fruit of the species.
Conservation status: Thesium hispidifructum has an extent of occurrence (EOO) of 44 km 2 and an area of occurrence (AOO) of 12 km 2 . It is known from only four gatherings at two to three locations in mountainous areas (Blouberg and Mount Letsjume), surrounded by much-transformed low-lying areas (Vivo and surrounds). As the most recent collection/observation of this species was in 1991, the state of these subpopulations is unknown, and needs to be verified during field studies. It is likely that the extensive human settlements and crop development in low-lying areas surroundings these mountains have already resulted in the loss of suitable habitat. Furthermore, expanding anthropogenic influences may easily push this very restricted species to extinction. It therefore qualifies for a threat status of Vulnerable (VU) under Criterion D2 [20].  Phenology: Thesium hispidifructum has been collected in flower and fruit in January, February, June, and July.
Etymology: The specific epithet hispidifructum refers to the hispidulous fruit of the species. Conservation status: Thesium hispidifructum has an extent of occurrence (EOO) of 44 km 2 and an area of occurrence (AOO) of 12 km 2 . It is known from only four gatherings at two to three locations in mountainous areas (Blouberg and Mount Letsjume), surrounded by much-transformed low-lying areas (Vivo and surrounds). As the most recent collection/observation of this species was in 1991, the state of these subpopulations is unknown, and needs to be verified during field studies. It is likely that the extensive human settlements and crop development in low-lying areas surroundings these mountains have already resulted in the loss of suitable habitat. Furthermore, expanding anthropogenic influences may easily push this very restricted species to extinction. It therefore qualifies for a threat status of Vulnerable (VU) under Criterion D2 [20].

Discussion
This study highlights the importance of fieldwork and the usefulness of information collected by both researchers and citizen scientists to aid taxonomy. Knowledge was generated through multiple collections made of a relatively poorly known Thesium species in the Waterberg. It was described by Brown in 1932 [18] using a cryptic diagnosis and was represented by limited material in herbaria. Through collections made over the past few months and photos taken of critical characters and uploaded onto iNaturalist, puzzle pieces were fit together to determine the identity of T. celatum and establish a better-informed, comprehensive morphological description. In addition, the collecting events stimulated further research and the recognition of a species new to science, T. hispidifructum, that would otherwise have gone unnoticed.
Both T. celatum and T. hispidifructum are perhaps most notable in their woody habits that grow up to 1.2 m tall and 1 m tall, respectively. Thesium species from the northern provinces of South Africa (Gauteng, Limpopo, Mpumalanga and North West Provinces) are predominantly herbaceous with only a handful (e.g., T. gracilarioides A.W.Hill, T. multiramulosum Pilg., T. procerum N.E.Br.) having woody stems that regularly grow above 0.5 m tall. Other uncommon character states in Thesium species from this region include the sometimes scabrid indumentum of T. celatum, its long leaves (2-22(-30) mm long), bracts (2-15 mm long) and flower lobes (1.2-2.5 mm long), as well as the hispidulous indumentum (including the flower and fruit exteriors) of T. hispidifructum. The undulate placental column shape, found in T. celatum, is unique in the genus (straight or twisted in other species).

Conclusions
An unidentified Thesium species from the Limpopo and North West Provinces in South Africa has been identified as the little-known T. celatum following an in-depth study of morphology and geographical distribution. Thesium celatum is easily distinguished from similar Thesium species by amongst others its tall, robust habit (up to 1.2 m tall), glabrous to scabrous indumentum, long flower lobes (1.2-2.5 mm) and undulate placental columns. At the same time, a further unidentified Thesium specimens housed in PRE were determined to be new and endemic to the Limpopo Province, characterised by amongst others its tall robust habit (up to 1 m tall), densely papillose to hispid indumentum, short flower lobes (0.8-1.2 mm) and straight placental columns. It is named T. hispidifructum after its hispid fruit, a rather rare character state in Thesium. This study once more alludes to the plethora of remaining taxonomic work needed in Thesium, a genus that has not been revised in southern Africa for almost 100 years as well as the usefulness of collecting activities and the help citizen scientists provide.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/taxonomy3010009/s1, Table S1: Specimen information of eight Thesium species morphologically similar to T. celatum and T. hispidifructum, examined during this study.